To adjust the amount of your pledge you simply click on the "Manage My Pledge" button. Kickstarter will open a page with your payment info and your pledge level. Next to the pledge level is a button to "Change Your Pledge". Click the button and add the amount you want to add to your current amount. You DON'T have to change your Reward Level if you are selecting add-ons and/or upgrading to the DELUXE EDITION, simply change the amount.
In the event that we cannot unlock a certain stretch goal (specifically the Moon, Coven Role, and Familiar Cards) they will be available for purchase during the pledge management phase. But, if they ARE unlocked, they are included for FREE with the game! So let's work on that by spreading the word!
Don't see the answer to your question? Ask the project creator directly.