Most of the time, when a tenant chooses to leave their property they will do so at the end of the fixed term of the tenancy and they will give you vacant possession of the property when they do.
However, sometimes one of your tenants will want to leave in the middle of a tenancy and the other remaining tenants will want to stay. In these circumstances if you do agree to allow them to leave the agreement, you can provide a new tenancy agreement that starts afresh, or you, the outgoing tenant, the remaining tenants and the replacement tenant can sign a deed of assignment. This will transfer the outgoing tenants interest in the property over to their replacement.
As a deed this document must be signed by all parties but that signature must be witnessed by someone who is not a family member or a party to the agreement. The most appropriate way to do this is to have the witness be physically present in the room as the parties sign the deed.