The Astute Manager can only log into one user account at any one time. If you change details or wish to change the account you're associating with the Astute Manager, you will need to click on the top right account menu icon and click on Sign Out. This will allow you to log back in using your updated account details.
Your account information can be changed by logging in to your Customer Account and going to My account > Contact Information. There you can click on Change email address, Change password and/or Update name before clicking Update to save. You can also do this from within the Astute Manager.