Hi,
Install Cubase 12 and Steinberg Activation Manager, please. Start Steinberg Activation Manager and login to your MySteinberg account. Click the Activate button next to Cubase 12 license. Done.
If you want to release the license from the old computer, start the Steinberg Activation Manager in the old system and click the Deactivate button there.
You can use one Cubase 12 license on up to 3 computers (even simultaneously).