You go to Details at the client you want to add more information and then go to Settings
From here you choose Settings, and can start adding new information.
In the Client Settings section, you have access to personalize your client and add information.
General (Name, Title, Hourly rate, Currency, Avatar Text, Avatar Color)
Details (Phone, Website, Email, Twitter, LinkedIn)
Address (Street, Zipcode, City, County, Latitude, Longitude)