Activating Cubase 10.5

Hi,

I am new to the forum and extremely IT illiterate.

I have been happily using Cubase 10 for the last 6 months. I have just installed version 10.5 but upon load I am asked for an activation code.

Where / How does one obtain the activation key when upgrading from existing older version?

many thanks for all your help
Rgds
D