How can I increase the contribution I have left in a Thankbox?
You first need to get to the message edit page. To do that follow these instructions.
Once you're on the edit message form look for the + Add More button where your existing contribution is shown.
After that just choose the extra amount you want to contribute, enter your payment details and your new contribution will be recorded.
Keep in mind that you cannot reduce your contribution amount. If you want to remove and get a refund for your contribution you have to delete your message.