Changing lease after lease term

[Christen Vogler]

If someone's lease is ending at the end of this month am I able to change something in it when it does end? It will automatically go to Month to Month status as of September 1st. What I am wanting to change is adding a month to month fee, for some reason this was not put into the lease when she signed it to being with. If I am able to change it how many days notice do I need to give her?