For one slot, answer No when Home asks whether you are working that day; the slot goes to another driver, and saying no costs nothing. To give up your whole line, ask the desk. The notice period is in your Schedule 1, and the release credit in clause 1.3 of the rental agreement.
What if I cannot work one of my slots?
Answer No to the day-before question for one slot; ask the desk to give up a whole line.