Fire up the launcher, click on the world name and select Edit. For renaming the folder, you just do that in your OS like normal.
If you're running an older version of Minecraft then Edit will be Rename.
Since when does Edit in the Launcher edit Worlds and World names?
The launcher Edits Minecraft VERSION profiles, which you can rename/edit.
Start your Minecraft profile from the launcher.
Select the world you want to rename and click "Edit". (don't click on the arrow to the left of the world name, that will start the game in that world.)
Edit the World Name.
If you wish to also change the folder name click on "Open Folder" and then highlight the world folder, right click, and click rename.
Close the File Explorer window.
Click "Save World" in Minecraft.
Thanks for the question. I had a world with a non-descriptive name and it's now got a name I like.
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