In order to make changes to your reservation, log in here and click "My Account" in the top left corner. From there, select the reservation you wish to modify:

Click "Edit" on the section you wish to change:

Select the sessions for your updated reservation, then click "Next":

Update the selection of nights you wish to attend, and click "Next":

Finally, review your updates and if they are correct, click "Proceed to Payment":

Your modifications will be complete once you arrive at a confirmation page. You should also receive an email confirming your changes.