The first step is to talk to your opponent and agree on a new date and time to play.
Once you've agreed on where to move the game, you need go league page and select schedule (ex. Leagues - Core - Schedule)
Then press RESCHEDULE BUTTON.
After that you need to choose date and time where you want to reschedule. After that press SEND SUGGESTION. When your opponent then accepts the reschedule request, the new date is set.