To manage your users, click on Users in the main menu:

On the Users page, you can:
click on the plus icon:

You will be taken to the "User create" page. As a minimum, you must enter:

Click on the view icon (eye) for the user to view their details:

You have an option to edit the user on the form.
Click on the edit icon (pencil) for the user to edit their details:

Click on the delete icon (trashcan) for the user to edit their details:

Edit a user (see above) and on the edit page, set the "Active" status to "False" and save the user:

The same above, but set the "Active" status to "True".