Keep everything you built, and let the new update's biomes and caves grow in everywhere you haven't been.
Drop your world here
Your world folder, or a .zip of it. It never leaves this device.
%appdata%\.minecraft\saves~/Library/Application Support/minecraft/saves~/.minecraft/savesIn Minecraft: Singleplayer → pick the world → Edit → Open World Folder.
Minecraft only uses the new terrain for chunks it generates for the first time. Everywhere you've already loaded is saved with the terrain of the version that made it. Clearing the chunks you never really used makes Minecraft generate them again — with the new terrain.
No. Every chunk has a counter of how long players were nearby, and anything you spent time near stays, with a border around it. Chunks with things you made — a sign you wrote on, a chest with your stuff, a pet, an item frame — stay too, and so do your bed, your position and the spawn. You also get a new copy; the original isn't changed.
Open the updated copy in a newer version than the one that made the world. If you open it in the same version, the cleared land grows back just as it was.
Not yet — Bedrock keeps chunks in a database this tool can't rewrite safely yet. It's coming. Java Edition worlds work now.
Minecraft adds to a chunk's counter every tick a player is within about 128 blocks of it. So five minutes means players were somewhere nearby for five minutes in total — not that you stood in that exact spot.