To unlink a Client from another Client File follow the steps below;

• Navigate to The Client File you want to unlink. 
• Scroll down to the Main Contact or Secondary Contact Section depending on who you are wanting to unlink. 
• Select the 'Person' Field drop down
• Select the option at the top of the list 'Select Existing Person'. 

This will unlink the Client File from the others it was linked too, you can see below that the person field is now not showing any other Client Files. 

You can now update the Client File.