choose a platter or tray that is bigger than the sheet you used. wedge a spaltula around the sides of the cake to loosen it.
place the platter , upside down on the cake and turn it over!
Use another platter to turn the cake back to front
hopefully the cake will come out without breaking or cracking, but a good frosting can take care of that as well. It is always good idea to use baking parchment
Good luck