Changing late payment penalty fees on a signed agreement

We have experienced a larger number of delinquent accounts in the last few months, we have in our contract a 1% penalty fee that is so low it is not even worth the time applying; it will not motivate anyone to pay their bills any faster. Can we change it to a 5% if we officially notify the clients with Certified mail? or what should be the path to follow. Clients with Net 15 terms continue to pay Net 30, 60 on whenever... Thank you.