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If you want to use Lean, the recommended way to install Lean is following these instructions under Regular install.

If you want to modify the code of Lean or the core library, the recommended way is to build Lean from source.

If you want to modify a single file in the core library (not the C++ source), then instead of building Lean from scratch, you can run (in your local lean/ directory)

git checkout v3.xx.x
git checkout -b my-branch-name
elan override set leanprover-community/lean:3.xx.x

You can now build the core library with lean --make library or open any Lean file is VSCode / Emacs and it will use the version of Lean you specified. You might have to restart Lean (in VScode: ctrl+shift+P Lean: Restart). Warning: all imported Lean files will be from the downloaded community version, not the version of the files in this repository. Therefore, this setup is not recommended if you modify more than one file. Moreover, editor features like Go to Definition will not behave correctly with this setup. For the best experience, build Lean from source.

Stable binary releases of Lean are available on the release page.