If you added a different account during onboarding, you can click the . . . on the upper left-hand corner and choose Select another email. A web browser will open that will allow you to choose the correct account to use with Insync.
You can also click Cancel add account if you don’t wish to sync the current account.
Clicking Quit will exit the app. The newly added account (ie alex.syncs@gmail.com) will be removed when you restart Insync.