Hello! I purchased this before I made an account, so how can I get an update (although i think that this website mentioned something about being able to tell if I did, so this might not be a huge issue)?
I believe if you don't want to use the Itch app, you can just re-download the zip file.
If you're using the Itch app, then it should be available in your library. Install it, and the Itch app will check out and update Mannequin automatically whenever a new release is up.