In the case of an unforeseen emergency, you are able to withdraw your accepted bid on an Op without penalty up to 12 hours prior to the start time for that Op. If you withdraw past the 12 hour mark, your reliability rating will be negatively affected which decreases your chances of being accepted to future Ops. If you withdraw within 4 hours of the Op's start time, you will receive a 1-star rating which could decrease the likelihood of bid acceptance.
Please go into the Op details of the Op you were accepted for and select "Withdraw". There will be a screen that pops up prompting you to view the effect of the withdrawal on your reliability rating and to select a reason for the Withdrawal.