If you added a different account during onboarding, you can click the 3-dot menu on the upper right-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.
Quit will exit the app. The newly added account (ie firstname.lastname@example.org) will be removed when you restart Insync.