Turning on cheats in a minecraft world that's already made

Java or Bedrock? Bedrock is easy: world settings, Game, flip Activate Cheats on. It'll warn you that achievements get switched off for that world, and that's permanent. Java: load the world, pause, Open to LAN, turn Allow Cheats on, start it. That only lasts for the session, so you redo it every time you load in. Making it stick means editing the world file with an NBT editor, which most people don't bother with.