If you made the wrong proposal (ie you did not select the correct time), please submit a support ticket on the website. Be sure to file your support ticket under the correct type, ie League.
If the other team did not accept your proposal, an admin will be able to cancel it and you will be able to resubmit one after.
NOTE: If the other team already accepted the schedule, you will need to agree on a reschedule time with the other team. The admin WILL NOT be able to reschedule the match unless both teams agree.
This system is designed to prevent abuse by preventing a team from offering and rejecting a time continuously to disrupt the scheduling process.