If your slot begins later than 24 hours, you can cancel it yourself in Staffomatic.
To do this, please click on the red cross as seen below; this will automatically un-assign you from this slot.
If your slot is in less than 24 hours, you will need to follow the process of trading your slot with another Stuwie (more info on trading slots here). If your slot has not successfully been traded you are still responsible for covering the slot.
NB: no-showing to your slot will have a negative impact on your CPS. More info on the no-show policy here.