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 the information for that user.
Caution when editing a user account - If the user's role changes and you need to change role types you do not want to change a user's role type. If you change the role type you will lose the history from the previous role type, as well. The only exception is a role type allowing admin access role type to another admin access role type. As long as they are still being given access to the backend they can be moved. Be sure to save when you have completed your changes.