1. Go to Web > Web Payment Settings
  2. Click on the Language tab
  3. Choose Payment for the Category.
  4. For item name "field to enter discount code," change the words in the text box from Discount code: to whatever you would like to label it.
  5. Click Save.

Note: For steps on how to hide the discount code from online forms (if you do not offer online promo discounts), follow the steps in this Knowledgebase.