Amending a rental greement

[Roy Lam]

I have a tenant who has been late on rent payments twice in five months, this month she still hasn't pay all of the rent for September.  I charge her $50.00 late fee after five days.  So she rather pay the late fee and be late for so long.  I would like to amend the rental agreement and raise the late fee so high that she would pay on time.  My question is, can I change the contents in the rental agreement while it doesn't expire until February 2020?  If so, what is the maximum I can charge for late fee?