How can I update to Ubuntu 21.04? [duplicate]

Today is the release of Ubuntu 21.04 (Hirsute Hippo). And I can't update to this version.

And also I selected ‘Notify me of a new Ubuntu version’ and changed it to ‘For any new version' on the Updates tab on Software & Update, but it's still not working. In addition, in the terminal, I've written:

sudo do-release-upgrade -c

And output was:

Checking for a new Ubuntu release
No new release found.

What should I do?

The release of the ISO has only just occurred; the update from prior releases doesn't get opened until a decision is made on the stability of the new release (watching bug reports etc) & a decision is made to turn the taps (where people are offered the upgrade). I'd recommend being patient and waiting until you're offered the upgrade.

You can use -d to force... but refer to the Ubuntu 21.04 Release Notes where you'll note

Upgrades from Ubuntu 20.10 to Ubuntu 21.04 are not enabled as it is possible for some systems to end up in an unbootable state if they use EFI version 1.10 - bug 1925010. Release upgrades will be enabled once an updated version of shim is available which is compatible with EFI version 1.10.