From the Inbox, touch and hold the Avatar icon (located to the left of the appropriate contact).
Tap the Edit icon (located in the upper-right).
If preferred, enter additional info (e.g., Email address, Organization, etc.) then tap Save (located in the upper left).
To change the account type, tap the Dropdown menu then select the desired account (e.g., Phone, Google, etc.).
Depending upon the account type, the options presented may vary.
To add more fields: