You should go to the project page and click on 'Manage pledge' and add +$25 on the 'pledged amount'. You only have to change the 'pledged amount', not the reward selection.
Here is an example: If you have pledged for the reward 'The fantasy kit', which is $59, you have to write down $84 on the ‘pledged amount’. And that's all ;)
Don't see the answer to your question? Ask the project creator directly.