Find contact and click on name to go to details page
All details on left sidebar are editable under 'About this contact' and 'Details'. Friendly Tip: Hover your mouse anywhere in the left sidebar and scroll down. It will show you more properties to edit!
Click 'Save' in bottom left screen once you've edited a property
Go to contact page of the contact you are wanting to create a deal for. On the top right sidebar, click 'Add' next to Deals section. See below reference