If you'd like to change which admin account is being used on any backup task, you can reauthenticate at any time from the Homepage tab. Make sure to sign out of O365 in your browser first before making this change; that way you can be prompted to choose an account to sign in with.

This can be done by clicking the name of your Exchange, SharePoint, OneDrive, or Teams backup under Homepage, then clicking this green area labeled "Connected.” 

From the next page, you can click “Authenticate” in the bottom right corner, then log into Microsoft with the account you'd like to use instead. The service account currently being used for that backup will always be grayed out, but after signing in with the new account, this field will update automatically.