Once you add a user, you can still modify them. If you have many users you need to modify, add info to, or delete, then take a look at the article about user uploads. However, if it's just a few, then it's easier to just do them one at a time.
First, navigate to the Admin section using the Application Switcher in the upper right. From there, click on Users & Permissions and in the menu that appears, click Manage Users.
Here, you will see all of your existing users. You can search for the user you want to modify or delete using the search box in the upper right.
To delete the user, click the X to the right of the person's name, and confirm you really want to delete them.
Note: Users can be restored later, and their records will come back with them.
If you want to modify the user instead of delete them, simply click on their name, then click on Edit User in the upper right. That will bring you to a screen where you can modify any existing info or fill out other fields with new info.
Once you're done, click Save.
Adding users one at a time