The “Upgrade this instance” button has had issues for a long time. So much so that its functionality was removed in later versions and now it links to the update guide.
Since you’re on Linux, login to your machine as root and run getamp update. Note this will stop/restart your running instances.
If you’re using a VPS, you’ll need access to the terminal for your VPS.
Refer here: How to update AMP to the latest version