ChessBase: Wiki

This is how to activate the games update for the Mega Database:

•  log in at ChessBase 18 (17/16) with your ChessBase Account name (don't use an e-mail as login),
• click on "New Games",
• select "Update Megabase [year]"
• and then click on "Activate subscription".
• In the upcoming dialog enter your Mega serial key and the additional four digit captcha.

https://en.chessbase.com/post/mega-database-a-year-of-weekly-updates

This tutorial shows the activation of the Mega2021, but the process is the same for later versions .