If the contact is already on the system it can be done from the client page.

If the details for the main contact have changed then you just need to go to the primary contact and select the pencil button and edit the contact details.

If you need to change the main contact then the best way to do this is to +Add Contact

If the contact is already on the system then they will show up automatically

Otherwise you will need to select + Create .....

Once you have put the new contact in or selected the existing contact, press Add Contact - they will then show up below the Primary Contact on the Client Page.

To change the primary contact, click on the new contact - four boxes will come up. The first one will change them to the primary contact and this is the one to click.

This will then change the contacts around

Your new contact is now the primary one for the client.