Need to do formal rent increase

Hi all

My tenant has been in my let house now for over 3 years and I have not done a rent increase in all that time. I have now informed her that the rent will be going up. She has an AST when she moved in for 6 months and then it just went onto a rolling tenancy agreement. How do I go about doing the
paperwork for the rent increase? I presume I don't have to prepare another tenancy agreement?

Please can someone advise. thanks