If you have agreed on a new time for the match with your opponent you have to change this on the ESL webpage.

The system automatically closes a match which not has been a result entered for within 24 hours after the date and time of the match.
So if you want to postpone a match you go to the match detail page where you normally enter the result. There you find a button for "postponing" the match.

The match will be set to "challenge" again and your opponent has to agree on the new time.