How to change the game mode in a downloaded minecraft survival map?

Ok, here's my problem: I downloaded this survival map for minecraft. I need to change this map to creative. The problem is, though, that whenever I try doing /gamemode 1, I get a message that says I don't have permission to use this command. Is there any way around this? All helpful answers will be highly appreciated.

Unfortunately, you can't. A pre-made server can only have cheats enabled or disabled by the original owner.

This is Quite simple. First, Go to your lan options. Second turn on lan world be sure to allow cheats. Third use the /gamemode 1 command BTW this only works in single player if you didn't already know