If you want to modify your pledge, but are unsure how to do so, please read the following instructions. When changing your pledge, do not hit "Cancel Pledge".
Visit the project page and click “Manage Your Pledge.” Enter a new amount in the pledge amount box. Note that you are not adding to your existing pledge; the amount you enter will be the total amount collected if the project is successfully funded.
http://www.kickstarter.com/help/faq/backer%20questions#HowCanIChanMyRewaSele Visit the project page and click “Manage Your Pledge.” To change your reward, make a new selection. Note that changing your reward selection does not automatically change your pledge amount.
These guidelines are borrowed directly from Kickstarter's official FAQ's.