To modify contact,

1. Open application
2. Go to contacts and click on contact to modify
3. Click on “Modify contact“
4. Modify details and click on “SAVE CONTACT” button at bottom of screen.

To delete/remove contact,

1. Open application
2. Go to contacts and click on contact to modify
3. Click on “Modify contact“
4. Modify details and click on “DELETE CONTACT” button at bottom of screen.

Note : when you delete contact it will move all messages to native/default messaging application of that contact.