PHP 7.2 too low, PHP 7.3 too high

[agisthos]

I am on 2.35 and when trying to update I am told PHP 7.2 version is too low

And when on PHP 7.3 am told cannot update because PHP version is too high

What to do? 

[agisthos]

Why do no images show up in posts here? When editing the post they show, but not when posted.