To edit your own User, click on your Username in the top navigation bar as per the screenshot below:
The Edit User dialog will allow you to upload a profile photo, make changes to your first name, last name, and email address, and add an Email Signature:
If you're an Account Owner (or have the necessary permissions) needing to add a new User or edit an existing User, navigating to Account > Users will allow you to click on the pencil under the Actions column to edit the User:
When on the Edit User dialog, you can do everything listed above, but you will also be able to edit the User's User Type and control whether or not they can see margins (provided it's included in your plan).
If you've reached the maximum number of Users included in your plan, please contact firstname.lastname@example.org to discuss custom plans.