Select a file in Finder. Open the Inspector panel for that file ("Get Info" in the context menu, or Cmd-I). If necessary, expand the "Open With" section. Observ

Select a file in Finder. Open the Inspector panel for that file ("Get Info" in the context menu, or Cmd-I). If necessary, expand the "Open With" section. Observe the presence of a "Change All..." button. It's not particularly discoverable, but I think that's mainly because Apple's own UI designers have all but forgotten the NeXTSTEP UI heritage, so doing a Cmd-I on things is not as useful a habit as it once was.