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In single player you can set it via the option button you see when you open the inventory. You need to use the global setting to get cheat mode activated.
As for multiplayer you would need to have op privileges. Outside of that you'd need some sort of hacking program which I hope no one here would help you with.
If you're in Singleplayer you can go to the MC menu, 'Open LAN world' (or something like that, haven't played MC in a long time), then choose to turn cheats on, now you start the LAN world.
Then you type in the chat /gamemode 0 and now you're in creative, do /gamemode 1 to get back to survival mode.