Are you wanting to be able to see what their payments will be if they make an extra payment in a given month?

If so make a basic payment calendar in months showing what their payments are (Based on a minimum payment) to pay it off within agreed terms.
Allow a field to add additional payments if made so that it is subtracted from the balance, this will in the end decrease your last payment(s) and/or term length depending upon how much is added.

Made a basic sheet as an example

QuickLoanCalculator for Payments.xlsx