Updating a tenancy agreement

Other

I have one tenant on Housing benefit. She is often late with her payments so I am organising for the local Credit Union to take over my payments. This means a drop slightly in rent but at least ill get it. She is also one month behind, which can be taken care of with her deposit. I wish to use the deposit to take care of the arrears, start with the credit union and keep an otherwise good tenant in her home. So my question is that I need to give her an amendment of her tenancy nd how do I do that without a new 6 month fixed period (she's on periodic now).. Can I just give her an amendment showing no deposit held? Can I serve her a new section 21 notice at the same time to account for future problems (she has an existing one which will become null and void, which she signed on entering the property.

Many thanks