After you submit your offer, it will appear on the task page. You can modify or withdraw your offer if it hasn't already been accepted.

If you wish to change your offer, choose the Update Offer button and set a new price. Proceed by clicking Continue and then Submit Offer. We recommend that you let the poster know why you adjusted your offer by leaving a comment as well. 

Select the Withdraw button next to your offer to withdraw it. You won't be able to take on the task any longer.

You won't be able to alter or withdraw your offer after the task has been assigned.