In the contacts tab, click the plus icon in the top-right

 

Click Create a custom contact

 

Enter the name and phone information required for the contact and click Save

 

To edit a contact, hover over the contact with your mouse cursor and click the i icon

 

Click Edit contact

 

Then make and save your changes

To delete a contact, right-click on the contact entry and click Remove from contacts

 

Confirm by clicking Remove