are you on a computer or a phone or tablet ?
if you are on a computer go to www.gmail.com
on the top right is a little icon with eaither your picture or a blue shadow of a person kind of 2 circles
click on that
it will give you the option to log out
log out and then you can log back in to the proper account
if it is on a phone it would be more complicated but i can tell you how to do it. but i need more information
if the above instructions are not clear enough let me know. I can get you screen shots or even log in remotely to do it with you.