I want to downgrade to Hugo 0.17 since one of the themes only supports on that version. How can I do this? Can i do it with homebrew?
You can learn more about installing software on your computer using the Homebrew docs.
If you intend to run multiple versions of Hugo, you are going to need to learn how to manage packages on your operating system. If you only need 0.17, grab a copy from the release page, and install it as normal for your OS.
Quick thought experiment: since you don’t really know how installing packages work, do you really want to be at the mercy of an older theme that hasn’t been updated in a year? Maybe pick another theme from the showcase.