How Can I Apply an Unapplied Payment?

In Sage 500 ERP - I'm looking at AR Payments and discovered we have some unapplied payments.  I'm not sure how to apply them to an invoice.  For example, we have $25 unapplied under one customer, when it really should have been applied to a different customer.  Sounds like a very simple issue to resolve, but I don't know how to fix it.  Please help.