From the admin dashboard's Home page, click the first icon at the top left corner, "Setup Users". To search for a user, enter one or more search criteria on the Setup Users page under the Search By section and click on the Search Button. All the users matching your search criteria will appear below on the user table.
From the displayed results click on the Edit Icon to the left next to the user you want to edit. This will populate that user's details in a new screen, where you can edit any information for that user. Be sure to save when you have completed your changes.