If you would like to choose a different reward level or if you want to increase your pledge for extra books, this is how you do it:
1) Go to the main project page. In the green "You're a backer!" box click the blue button that says "Manage".
2) On the next page, click the button that says, "Change your pledge". You can either increase your current pledge amount OR change your pledge level.
3) Once you have edited your pledge amount or level, click "Continue" and then "Confirm". That's it! Drop me a note to let me know what the extra funds are for. :)