Mistakes happen. We're human, after all.
If you have applied for an available shift that you are not able to work, then you can cancel your application if a manager has not yet approved it.
Click on the available shift you applied for in the schedule, and you'll see the option to retract your application:
If you've already been approved for the shift, then you may either have the option to make the shift available, in which case you'll still be responsible for it until someone else takes it over.
If you do not have the option to make the shift available once it's been assigned to you, then you will need to contact your manager directly to let them know you cannot work the shift.