H'llo!
Try this!
open your dialer or contacts (Where your contacts are located)
Find the contact your going to edit.
Long press on their name till a menu pops up.
On this menu you see things like delete, forward etc.
Find Edit contact... Or just edit.
Click it, Now you can edit this contact.
If you want to add an a phone number look at the phone tab
You will see the number you originally typed below it is an add new. Just click the "Phone" box and type the number.
After your done make sure you click on save or done.
Hope that helps!