1. Open the recurring gift
  2. Select the Amendments tab
  3. Click New Amendment
  4. Click schedule and edit the payment schedule 

Alternative solution for versions prior to 7.61.790:
  1. Mark the recurring gift as completed
  2. Create a new recurring gift
    Note: A recurring gift should not be thought of as a pledge since there really isn't a pledge amount and a balance, etc.  It's just a new gift over and over.