You can edit a contact by going to your contacts list from the features menu.
A list of your existing contacts will appear. Groups are also available hear in the side tab "Groups".
Click on the contact/group your would like to edit.
When you edit a contact:
You will see the entire contact information form. Change the appropriate information and click save.
When you edit a group:
Click on the group you would like to edit. You can now change the group name, add, and remove contacts from the group. Click Save when you are done.
For both individual contacts and groups you can swipe to the right to delete the contact/group.