Create change-default-version.md

cheatsheets/python/install/change-default-version.md

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+# Change default version
+
+## macOS
+
+If you have multiple versions installed with Brew, you can set the new one as your default:
+
+```sh
+$ brew unlink [email protected]
+$ brew unlink [email protected]
+$ brew link --force [email protected]
+```
+
+```sh
+$ python3 -V
+```
+
+```sh
+$ which python
+```
+
+If another Python is picked ahead of the Brew versions, check the order of values in `PATH` and set in your shell config.