Changing your PHP version is a premium hosting functionality and is not available in free hosting.
That being said:
As long as Joomla supports running on PHP 7.4 (the current version) you can safely ignore this warning. If this ever changes, you’ll just need to remain on an older version of the software that supports it for a while.
A migration of free hosting servers to PHP 8 (specifically 8.1) has been planned for a while but has ran into some troubles, all developments about it will be shared on this thread: