One of the sections within the Know Your Client area of contact records is the Personal Interests section. Note: unlike much of the Know Your Client information, Personal Interests are not accessible from a contact's Overview page. You would need to first click Know Your Client from your Left Menu Bar:
Then, Personal Interests are accessible by clicking "Misc Client Info" on the left-hand side of this section:
To add Personal Interests, just click the Add button in the top right corner of this section. You'll then see the below:
Here you can type in a New Interest (one that hasn't previously been assigned to a contact in your database) or check off the radio box(es) for any existing interest(s) that you'd like to assign to the contact. After making your selection(s), simply click add.
To later remove a Personal Interest from a contact, just come to this same area, click the corresponding Actions menu for that Interest and then click "Remove Interest":