From the Ubuntu 24.04 Release Notes:

Upgrades

Users of Ubuntu 23.10 will be offered an automatic upgrade to 24.04 soon after the release.
Users of 22.04 LTS however will be offered the automatic upgrade when 24.04.1 LTS is released, which is scheduled for the 15th of August.

"Soon after the release" typically does not mean a day or two. Historically, it has meant several weeks.

• It will be offered to 23.10 users when it has been tested and is ready. When the Ubuntu Release Team decides upon a date, they will announce it.
• It will be offered to 22.04 LTS users around 15 August 2024.

Non-LTS users are sometimes surprised by this LTS-specific delay in relase-upgrades. The much larger numbers of LTS users has led to a phased rollout of LTS releases so that the community and developers can offer the best support as inevitable problems are discovered.

• Impatient folks can experiment with a release-upgrade by using do-release-upgrade -d, which still works for a week or two until the next development release repositories are set up. However, be warned that it might fail. You may encounter unexpected behavior. Your system may become unbootable. You may lose data. The use of -d means "I want to be a tester" and that you are willing to accept risk.

If you don’t want to wait till August 2024 to upgrade to 24.04 and are ok with encountering some bugs following guide can help you.

https://www.cyberciti.biz/faq/how-to-upgrade-from-ubuntu-22-04-lts-to-ubuntu-24-04-lts/

There is a two step path to upgrade.

First upgrade to 23.10.
Then upgrade form 23.10 to 24.04.

Are you getting this error?

There is no development version of an LTS available.
To upgrade to the latest non-LTS development release set Prompt=normal in
/etc/update-manager/release-upgrades.

There are multiple ways to upgrade Ubuntu 22.04 LTS before the release of 24.04.1 LTS, scheduled for August 15th, 2024. Here’s one safe method:

Edit the /etc/update-manager/release-upgrades file and set Prompt=normal

Run:

sudo nano /etc/update-manager/release-upgrades

Set:

Prompt=normal

Save and close the file.

Next, run:

sudo do-release-upgrade -d

Follow all onscreen instructions. This will get you 23.10 release and reboot the system.

Run:

sudo reboot

Then, again edit the /etc/update-manager/release-upgrades and set Prompt=lts

Type:

sudo nano /etc/update-manager/release-upgrades

Set:

Prompt=lts

Save and close the file.

Finally, type the following command and follow the rest of the guide to upgrade from 23.10 to 24.04 LTS:

sudo do-release-upgrade -d

This note will automatically disappear after August 15th, 2024, as there will be no need for this kind of workaround. As a seasoned sysadmin and developer, I recommend waiting until the release of 24.04.1 LTS (scheduled for August 15th, 2024) before upgrading from Ubuntu 22.04 LTS. This ensures superb stability and minimizes potential compatibility issues with your apps. However, you can use these instructions for testing purposes. This is a great way to check if your applications will work seamlessly with Ubuntu 24.04 LTS.

Above answer suggests upgrade to 23.04 first, and then 24.04, however, there still have method to upgrade from 22.04 to 24.04 direct.
The upgrade program will check website record in /etc/update-manager/meta-release, in which, the ubuntu 24.04 is marked as supported: 0 logically, you may enable it by point to a custom modify file.

Below an example to use a temporary local server to do this replace.

1. mkdir /tmp/fake-ubuntu-release-meta
cd /tmp/fake-ubuntu-release-meta

2. create a modified meta-release file

cat <<EOF >meta-release-lts
Dist: jammy
Name: Jammy Jellyfish
Version: 22.04.3 LTS
Date: Thu, 21 April 2022 22:04:00 UTC
Supported: 1
Description: This is the 22.04.3 LTS release
Release-File: http://archive.ubuntu.com/ubuntu/dists/jammy-updates/Release
ReleaseNotes: http://archive.ubuntu.com/ubuntu/dists/jammy-updates/main/dist-upgrader-all/current/ReleaseAnnouncement
ReleaseNotesHtml: http://archive.ubuntu.com/ubuntu/dists/jammy-updates/main/dist-upgrader-all/current/ReleaseAnnouncement.html
UpgradeTool: http://archive.ubuntu.com/ubuntu/dists/jammy-updates/main/dist-upgrader-all/current/jammy.tar.gz
UpgradeToolSignature: http://archive.ubuntu.com/ubuntu/dists/jammy-updates/main/dist-upgrader-all/current/jammy.tar.gz.gpg


Dist: noble
Name: Noble Numbat
Version: 24.04 LTS
Date: Thu, 25 April 2024 24:04:00 UTC
Supported: 1
Description: This is the 24.04 LTS release
Release-File: http://archive.ubuntu.com/ubuntu/dists/noble/Release
ReleaseNotes: http://archive.ubuntu.com/ubuntu/dists/noble/main/dist-upgrader-all/current/ReleaseAnnouncement
ReleaseNotesHtml: http://archive.ubuntu.com/ubuntu/dists/noble/main/dist-upgrader-all/current/ReleaseAnnouncement.html
UpgradeTool: http://archive.ubuntu.com/ubuntu/dists/noble/main/dist-upgrader-all/current/noble.tar.gz
UpgradeToolSignature: http://archive.ubuntu.com/ubuntu/dists/noble/main/dist-upgrader-all/current/noble.tar.gz.gpg
EOF

3. vim meta-release-lts -c "set ff=unix" -c ":wq"
   Convert it’s line feed to unix style, instead of windows style

4. python -m http.server
   Start a small web server locally for the file above

5. Open a new terminal, edit /etc/update-manager/meta-release, change it to

[METARELEASE]
# original
#URI = https://changelogs.ubuntu.com/meta-release
#URI_LTS = https://changelogs.ubuntu.com/meta-release-lts

# modify
URI_LTS = http://127.0.0.1:8000/meta-release-lts

URI_UNSTABLE_POSTFIX = -development
URI_PROPOSED_POSTFIX = -proposed

5. In the new terminal, run
   do-release-upgrade

6. Boom! Now just go through the normal upgrade process.

7. Dont forget to recover the file content after upgrade.

Warning: This method is a temporary solution, it shall not be needed after the 24.04 upgrade become GA.