PHP 7.4 has end of life on 28 November 2022, so it won't get security updates any more. I suggest to follow my instructions to avoid 500 Errors on your joomla 3 or 4 sites when updating to PHP 8.0:
- backup your site
- set Global configurations/Server/Error reporting to Maximum
- Search for updates, and update the joomla extensions and joomla to the latest 3.10.x or 4.x
- Test your site
- Update PHP to 8.0 (not 8.1), e.g. in cpanel if you have access to it
- Increase the php memory_limit to 128M, e.g. in cpanel or in a php.ini file
- test your site
- if you see an error message, investigate which joomla extension caused it. If it's a memory size problem, try to increase the php memory_limit, or disable that extension. Maybe you need to switch back to php 7.4 to be able to log in to the admin area if you want to disable an extension. Then start again from number 5.
After testing switch back Global configurations/Server/Error reporting to None.