Installing the updated beta replaces the Office installed one. I don't know if it's possible to have the old Office installed one and the beta client at the same time.
Yet when I search for teams in the start menu, I get two entries for "Microsoft Teams (Work or school)". If I click on the one without "new", it'll launch the old one and ask me if I want to go back to the old one or keep using the new one.
I think you can uninstall the old one but by default just enabling the preview doesn't uninstall it. Or at least, that's the behavior I've seen on my machine (same as vladvasiliu).