Currently, you can't manage contacts from the mobile app - things such as changing their number or the contact name.
If you need to make changes to contact, login to the web dashboard at http://dashboard.ninjanumber.com.
After logging into the app select, on the left-hand side select Contacts 
You will then see Contacts.
Select "Contacts"
To the right of each contact is an icon of a Pencil to edit or a waste bin to discard the contact. 