Joint tenancy - 1 leaving, 1 new tenant

Tenancy Types and Management

I currently have 2 tenants who are not getting on. One wants to leave (and is requesing deposit returned and to be taken off teh agreement)and the other can find a replacement.

What should I do with the existing Tenancy agreement which runs until November? Should I end the existing agreement early - if so do I serve any special forms or will an email be sufficient? Then draw up a new one for the remaining tenant and the replacement one for the last 3 months, or can I just write a memo and attach it to the existing agreement?

Or should I sign the existing tenancy over to the remaining tenant? And let him sort out any agreement with the replacement.

I am conscious that I need to follow correct procedure

Any advice would be appreciated