The CE to EE update guides are for subscribers of the Enterprise Edition only. The steps are very similar to a version upgrade: stop the server, get the code, update config files for the new functionality, install libs and do migrations, update the init script, start the application and check the application status.
Upgrader is an automatic ruby script that performs the update for installations from source.
Patch versions guide includes the steps needed for a patch version, eg. 6.2.0 to 6.2.1.
Miscellaneous
MySQL to PostgreSQL guides you through migrating your database from MySQL to PostgreSQL.
MySQL installation guide contains additional information about configuring GitLab to work with a MySQL database.