If you launch the Backblaze application/ program, you will be able to see email address of your account.
Mac: Go to the menu bar icon, and select Backblaze Preferences
PC: Go to the notification area or the system tray, and select Backblaze Control Panel
Once the Preferences/Control Panel is open, you can look in the top right of the window to see what account is tied to this installation.
If you would like to change the email address associated with your account, here's how to do so:
1. Sign in to https://secure.backblaze.com/user_signin.htm with your current Backblaze account email address and password.
2. Choose "My Settings" from the navigation on the left hand side.
3. Select "Change Email Address"
4. Enter in your new email address