To edit your availability, go to the My Availability page (accessible by clicking your name in the upper right). Use the arrows at the top to navigate to the week / day you need to edit, and select the time that you need to edit. You can either update the times and Save, or you can Delete the availability completely.
NOTE: If you input a series of availability (i.e. repeated the same availability over multiple days or week), when you edit the the availability, you will have the option to Save or Delete JUST THIS DAY by clicking on the text for Save or Delete OR you can save or delete for this day and everything in the series following this day or the entire series by clicking on the arrow next to the button (see below).