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 or withdraw it.
To change your offer, select the Update Offer button and enter your new price. Remember to select Continue and then Submit Offer to confirm the amount. It's also a good idea to leave a comment for the Poster about why you have changed your offer.
To withdraw your offer, select the Withdraw button next to your offer. You will no longer be able to be assigned to the task.
Once the task has been assigned, you will not be able to change or withdraw your offer price.