Search for the contact that you'd like to update in the "Jump to contact.." search bar and click on their name.
You will be brought to the contact's Dashboard. In the yellow sidebar, on the right side of your screen, find the Assigned Staff section.
Select the (edit) option next to Assigned Staff.
From the box that opens in the current window, select the staff member you want assign to this contact.
To remove a staff member, select the (unassign) option next to the staff member name. Changes are automatically saved.