Don't have time to carry out a mission you've booked? You can cancel your mission directly in the app. Simply click the 'give up' button in the bottom left part of the screen in the first page of the mission. You can also let your mission expire, but mind you'll lose more XP by doing so.
By abandoning a mission manually you'll lose 50 XP and if you let your mission expire you will lose 150 XP, so it's always better to abandon it manually!
Once you have cancelled your mission it will become available for other Eyes again, so if you know you can't complete your mission it's always best to cancel it immediately, so someone else can book it.