If the only problem you are having with your fan is that it will not go in reverse, then it is likely an issue with the switch that puts it in reverse.
Problem with the remote
If you are trying to put the fan into reverse with the fan and it won’t work, try holding down the reverse button for several seconds. It can take time for the fan to make the switch here, so be patient.
Problem with switch on fan
If you have a switch on your fan that is supposed to reverse the fan, then the problem is likely that the switch on the fan is bad and needs replaced. To get the switch contact the store you bought it from or the company that makes the fans and ask about it. Once you get the new switch all you have to do is dissemble the fan and remove the old switch by untwisting all of the connectors to the switch and put the new switch in.