The primary contact is the main contact for the deal and company. It shows with whom you are mostly involved in the communication.
Update the primary contact for the deal
Do follow the below steps to add and update primary contact -
1. Go the deal list.
2. Choose a deal, go to the Details tab, click on edit icon & there you will see an option of "primary contact" from where you can add a new primary contact.
3. If a person is already in your list, then as you write the name you will see people in drop-down list else you will have an option to create and add a primary contact to the deal.
4. To update "primary contact" , go to the related tab, from here you can update the primary contact.
In below screenshot, you can there is already person related to this deal from which one Mark Taiwan is the primary contact.
From here, I updated the primary contact. I updated Mr. Andrew Kramer as the primary contact.