Once you've requested additional fees from a guest, you're able to go back and edit or cancel your request. You can edit or cancel your request immediately after sending it, or at any point before the guest accepts your request. Note that once a request for money has been cancelled, you will not be able to request money again.
After you've sent your request for money, you'll be brought to a confirmation screen:

From there, click Edit to modify your request. This will allow you to modify the requested fees and the explanation, as well as upload additional evidence.
To cancel the request, click on the Cancel request button. A pop-up window will open asking you to confirm your decision:

Click on Cancel request again, and the request will be cancelled.
To edit or cancel a request for money after you've exited the confirmation screen, simply access the booking and click on the banner at the top of the page:

You'll have access to edit or cancel the request from there.