To update the contacts for your service, log into www.thejag.org.uk.
Under 'My JAG' in the top right corner, select 'My Unit Information'
You'll then see the page below. You can edit existing contacts or add a new member of staff. You always need to have a Medical Lead, Nurse Lead and Management Lead, so you won't be able to unselect a box. Instead, add the new unit contact and swap the lead role over to the new contact.