HTTP 500 Error after trying to update to Craft 3.1.1
The error message indicates your php.ini file's memory_limit setting is still sitting at 256M (268435456).

Double check you've bumped it up to 512M and you're editing the correct php.ini file for the site.

From there, the quickest way to get the site back up and running is if you have Composer installed from the command line, to nuke your vendor folder and composer.lock file, then run composer update from your project's root to pull in a fresh set of Craft's dependencies.

To be safe, you'll probably also want to restore your database from a pre-update attempt database backup as well (you can't be sure at what point the original fatal error occurred during the update process).