On a multiplayer server:

To turn on cheats:

1. Type op <Your username> in the console.
2. Done.

To turn off cheats:

1. Type /deop <Your username>
2. Done.

On a singleplayer world:

To turn on cheats:

Using the built-in LAN server:

1. Pause the game.
2. Click on Open to LAN.
3. In the options, change Allow Cheats: false to true (click on it).
4. Open the server.

Changing your save file:

1. Close your world.
2. Using NBTEdit, open your save file (%AppData%/.minecraft/saves/<World Name>/save.dat)
3. Change the NBT node AllowCommands from 0 to 1.
4. Save and reopen the world.

To turn off cheats:

Using the built-in LAN server:
Note: This only works on players who connect to this world, not you.

1. Pause the game.
2. Click on Open to LAN.
3. In the options, change Allow Cheats: true to false (click on it, if it already displays Allow Cheats: false, obviously don't click).
4. Open the server.

Changing your save file:

1. Close your world.
2. Using NBTEdit, open your save file (%AppData%/.minecraft/saves/<World Name>/save.dat)
3. Change the NBT node AllowCommands from 1 to 0.
4. Save and reopen the world.