1. Click on the Manage my Pledge button. (this is right below "days remaining" on the upper right hand column)
2. In a box you will find the dollar amount that you've pledged. Change that to desired amount. If you are adding an item, look for the reward level of that item and add that amount to your existing pledge.
3. DO NOT change your reward selection.
4. Send a message to the project creator (NOT in the comments section, but as a private message) and tell the add on item(s) you would like.