Once you've placed your offer you will see it listed on the task page. If your offer hasn't yet been accepted, you can update your offer by selecting 'Update Offer'. Remember to select 'Continue' and then 'Submit Offer' to confirm the amount.
If you have already been assigned to the task, you can't withdraw your offer.
In this case, if you're unable to complete the task and need to cancel it, select the 'Cancel Task' button on the task page. You will then be prompted through the task cancellation process.