You can download the Dev and Canary builds for the new Edge browser on Windows and macOS now! Head here for more info and download links.

The Beta channel isn't released yet but you can find the installer officially hosted via this link (https://c2rsetup.officeapps.live.com/c2r/downloadEdge.aspx?ProductreleaseID=Edge&platform=Default&version=Edge&Channel=Beta&language=en-us).

TIP: If you want to enable dark mode on the browser, make sure it's enabled for the OS then enter edge://flags in the URL bar and switch "Microsoft Edge theme" to "Enabled".