Visit the project page and click the blue “Manage" button that appears next to your pledge amount. Select "Update your pledge" on the following page. Then choose the new reward tier you'd like to pledge for and your pledge amount will update automatically. If you don't want to receive a reward, select "No thanks, I just want to help the project." and enter your new pledge amount.
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.