Requirements are on the home page
PHP 5.4 or newer
PHP 7.4 recommended
MySQL 5.5 or newer
MariaDB also compatible
Follow the upgrade steps as per:https://www.ubbcentral.com/doc_upgrade7.php
Including the two css files mentioned in the notes.
Pay attention to the screen prompts especially on the language files.
The upgrade has never been easier.
Now if you were going from classic then yes there would be some intermediate upgrades.
But you can do this in one step to 7.5.5
( sorry meant to say 7.7.5)
There also is a UBB test script if you wish to run before the upgrade at the UBB wikihttps://ubbwiki.com/latest/system-requirements-test