When you request a new monthly rent amount, an email will be sent to your renter notifying them of the change, along with a link to be able to accept it.
If your renter does not see this email, or accidentally deletes it, they can also log into their payments hub to accept the new change directly on the website here: https://www.zillow.com/renter-hub/payments
If your renter does not accept the change before the next due date, any payment sent will be for the old amount.