@Dwayne48 Do you mean, swap out one member of the group for a different person without changing the guest count? If the person from whose account the booking was placed is still staying there, then all you need to do this is ask the host's permission, and supply whatever information they request about the alternate guest.
However, it is not possible to swap the stay from one guest account to another. The person who placed the original booking is still responsible for it - and liable for any damage or incidents that occur. And if you attempt to "swap guests" without the host's permission, you invalidate the entire booking - the host would have every right to terminate it without refund.