Find the practices you have scheduled by clicking on TEAMSTAFF, TIMESLOTS. Then in the Search window Change STATUS to RESERVED and put in part of your team name next to UseBy or part of your name next to ReservedBy (if you reserved the field). Then press show.
At the end of the line for each practice is a sign. Click on that entry for each practice you wish to cancel. You will get a confirmation screen for each one.