You can add a contact's information or modify their details on different parts of the desktop dashboard.
Editing a contact's information from the Inbox tab:
Go to the Inbox tab.
On the left-hand side of the screen, select a conversation with the contacts listed.
You will now see the conversation with that contact along with their details on the right-hand side of the screen.
In the "Contact Details" section, click on the pencil icon (

) to edit the contact's information.
Change the desired information.
When done, click "Save."
Editing a contact's information from the Contacts tab:
Go to the Contacts tab.
Under "Search Contacts," search for the contact you're looking for by name or number.
Once you find the contact you're looking for, click on their first name, last name, phone number, birth date or date added.
The contact's information will be shown on the right-hand side of the screen.
In the "Contact Details" section, click on the pencil icon (

) to edit the contact's information.
Change the desired information.
When done, click "Save."