Nextcloud version: 25.0.13 - 25.0.13.2
Operating system and version: Linux 5.4.0-174-generic #193-Ubuntu SMP
Apache or nginx version (eg, Apache 2.4.25): Apache (fpm-fcgi)
PHP version: 7.4.33
Database: mysql 10.5.25
The issue you are facing:
Sorry - tried to google - but no success. Simple issue, I can’t go beyond 25, since PHP 7.4. is not supported - but when I update PHP to 8.2 I can’t access the update page anymore 
You should do it step by step, first php8.0 then php8.1 etc.
Since you are on Ubuntu 20.04 (focal), you can use the → php-updater script ← for that. It is made to make life easy 
But I would seriously recommend that you switch to a newer Ubuntu first, focal is really no longer up to date in many ways. The extended PHP support for focal ends in 10 months. The current LTS version is 24.04 and will be supported until 06.2029, so almost 5 years from now.
Much and good luck,
ernolf
thx for your hints. Unfortunately, I can’t change the OS - it is on my hosters side.
I was now able to upgrade php to 8.0 - but Nextcloud still doesn’t offer an upgrade “your version is up to date” - I thought that was due to the fact, that 26 doesn’t support PHP 7.4 - but there must be another reason. Pls advise
Maybe you should change your hoster. Do you have a pure web space or a virtual server? You know that Ubuntu 20.04 LTS is now 4 years old? In between there have been 8 Nextcloud versions alone.
To save Nextcloud, I would personally back it up (Nextcloud system, data, database) and reconstruct it locally on a test system. Then update it there with full access until Nextcloud 29 and then get a proper hoster.
Alternatively, get a Managed Nextscloud, where the hoster not only takes care of the operating system, but also the Nextcloud. If only the data and not the rest of your Nextcloud is important to you, you can upload the data to a new Managed Nextcloud and start again there. But you will lost e.g. all shares.
Please check, whther really PHP 8 is in use now. On some systems PHP 7.4 remains still active after upgrading to 8.x.:
Login in browser as administrator
select Administratove Settings - System
scroll down to bottom
thx - upgrading PHP in smaller steps worked, but now I am stuck in Maintenance Mode. I have no sudo rights via ssh. How can I transfer the content to a clean installation now?
… but you should have the ability to edit the config/config.php file!
There you should switch
'maintenance' => true,
to
'maintenance' => false,
… or use the occ commandline tool. That does not have to be invoked by root but your webserver user.
Much and good luck,
ernolf
yes, I found this - now I am one step further: Error 500 (but the backend seems to work, it is synching) - discussing it here Error 500 after upgrade to Version 29.0.2 - #4 by jtr