If other users have granted you permission to access using their logins, a Login As option appears at the end of the Settings list:
Select this and choose the user you are to log in as:
After logging in as that person, their name will appear at the top of your settings list and you will be able to undertake any processing they are permitted to do. However, any data changes you make will be time stamped with your name.
After completion of processing for the other user, log out and then log in again under your own name.