This concerns only minor version upgrades, like 5.0.x to 5.0.y or 5.1.x to 5.1.y.
For major upgrades like 5.0.x to 5.1.y, see the upgrade notes.
If you want to upgrade your project to the last Diem release, you need to modify the file:
/* * Include Diem */ require_once '/path/to/diem/dmCorePlugin/lib/core/dm.php';
Here you tell your project where Diem is.
Just replace path/to/diem by the path to the diem-5.0.0_* directory.
Then, open a command line in your project root dir and run:
php symfony dm:setup