Hi Roan de Jong,
The upgrade process for a stack is the exact same for a standalone switch. The download/install will automatically happen on all switches in the stack with no additional commands needed. Be sure to reboot the entire stack at the same time to complete the upgrade, you don't want slots on different versions.
Some helpful links:
Specifically using the web GUI:
How To: How To: Upgrade the EXOS Operating System via Chalet (Web GUI) | Extreme Portal (force.com)
I hope that helps!