Unfortunately, if you have reserved a mission it's not possible to delete the reservation without losing XP. When booking a mission, you confirm that you are able to complete the mission and therefore, it's hidden from other Eyes.
If due to unforeseen circumstances you are unable to complete the mission, simply cancel it in your app by clicking the 'give up' button. Note that we'll deduct 50 XP if you decide to do so.
If you don't cancel the mission manually, the reservation period will automatically run out, which results in a deduction of 150 XP.
So, it's always better to ONLY book a mission if you're 100% sure you will able to complete it :)