Your contact's Overview screen contains a User Defined Fields (UDFs) area within the Know Your Client section for the contact. Below is an example of this section:
If you are on another tab in this area, it is accessible by clicking "UDFs" as highlighted above.
Clicking Add in the top right corner of this section will allow you to select new UDFs and Values to add to a contact.
Clicking the Actions menu for a UDF already assigned to the contact will give you options to edit or delete that UDF:
You can learn more about working with UDFs here.