Please see Update #15 for a step by step explanation with pictures. Below this is repeated without the pictures:
Suppose you live in the USA and pledged $65 for a copy of Fief. You now want to add the Templar and Tuetonic Knights Buildings Pack to your pledge. To do so, you will need to increase your pledge by $12, from $65 to $77.
1. Go to the top of the Fief Kickstarter page and click on the blue 'Manage Your Pledge' button.
2. A window will pop up showing the $65 amount you pledged and below this your Reward selection. You just need to increase the amount to $77. Do not change your Reward selection.
3. Now click on the green Continue to next step button at the bottom of the window. A Confirm your changes window will pop up. Click confirm and your new pledge level will be displayed. Now click on the green Continue to Amazon button. This will take you to your Amazon account, where you will give them the OK that you want to increase your charge.
And you are done!