This can be agreed in the written form of email or a document. If everyone agrees, then record the change in writing, have yourself and all the remaining tenants sign it & make sure everyone has a copy (landlord, the tenant who’s leaving, and the tenants who’re staying)
If the leaving tenant has contributed to the bond, complete and send a change of tenant form to Tenancy Services.
You can then upload copies of the paperwork to your tenancy on myRent for safe keeping.
Once your ready for your departing tenant to be removed from the tenancy you can do this on the people page of the tenancy by following the steps here
The new tenant can be onboarded to your tenancy through the people section of the tenancy.