You have two ways to do this:
1. Change the rent for a single month
go to the Tenant's Payments section
go to the payment in question
click on the pencil icon
a popup will open where you can change the amount of the specific one month
2. Change the rent for the next few months
go to the tenant profile, from the Tenants section
go to the Contracts Section (second button)
at the bottom right, you will find a button called Edit Lease
click it to change the amount