The easiest way to edit a contact is from your PC here:
It can be done from the contacts app on your phone too. Someone with you contacts app will chime-in with the necessary taps.
To edit a contact from the Contacts app, select the contact that you wish to edit. Next, touch the pencil icon located in the upper right corner. To save the edits, touch the check mark icon located in the upper right corner. Hope this helps.