Tenancy leaves early joint agreement

Tenancy Types and Management

Hello,
I have a AST with 2 tenants jointly responsible that is of one year effective from 1 august. One of the tenant notify me about leaving the property mid October, only after 2.5 months.
The break clause is 4 months with 2 months notice.
I read through previous similar topics in the forum that landlord should allow tenant to leave early on the condition suitable tenant move in. My questions are as follows:
- how to amend the existing AST which cover both of my tenants, is the signed letter of surrender sufficient and then I can add the new tenant in the existing AST via addendum to the existing one.
- is there any template for letter of surrender?
- what about the protected deposit where both names of current tenants appear?
Thanks for your help