You can find the link to your account management page from the "Thanks a lot for your purchase" email that was sent to your payer email.
If you are a Supermetrics for Google Drive user, you can also find the link to account management here:
For Data Studio users, there is also the Team Management tool that you can use to do some license management from. For more information, see Team Management Overview. (Note that you still must change payer details and renew in Account Management. These are not available in Team Management yet.)