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Below is a summary of the article.
Changing the Ruby version on macOS depends on how it was installed. First, find how it was installed (see Check Ruby version on Mac). Is it the pre-installed Mac system Ruby? Was it installed with Homebrew or a version manager? Don't try to change the "system" Ruby as it is there for macOS, not you. It's best to install a software version manager if you plan to switch among Ruby versions. Each version manager has a command to list and set Ruby versions. See the article Change Ruby version Mac.