Open up Steam and navigate to your LIBRARY.

Right click on the game in your library and choose "Properties."

Selected the "BETAS" tab.

In the text box to "Enter beta access for to unlock private beta:" enter the password "ineedlegacyaccess".

Click on the "CHECK CODE" button.

You should see a confirmation message that access has been granted.

If not already selected in the drop down, select "legacy" and click close.

Steam may require you to download and update for the game. Wait for this to happen or force it using "LIBRARY -> DOWNLOADS".

Once the game has been updated, click "PLAY" as usual and you will be running the legacy branch.