The only way to switch an existing world to Hardcore mode is to use a third-party app to edit the game's files. How to change game modes in 'Minecraft: Java Edition' 1.

Depending on the version being played, fans of the game are just a few short clicks from a fresh handle and new possibilities. How To Change Your Username in Minecraft Using Microsoft ...

Everything you need to know to install and play Snapshots in Minecraft Java Edition.