Once you make a pledge, a "manage my pledge" button will appear. Click that and follow the prompts to change either the reward or the dollar amount of your pledge. Do this after we have communicated and I've let you know how much to add for additional items.
Remember, Kickstarter doesn't process your credit card until the end of the project and only if funding goal has been reached. So changing your mind during the campaign won't mess with your credit card.