To access the Beta branch on Steam, please follow these steps:
1) In your Steam Library Right click on Pillars of Eternity II, and select "Properties"
2) Select the "Beta" Tab, and click the Drop Down for labeled "NONE - Opt out of all beta programs", and instead select "Beta - Public Beta branch for upcoming patches"
3) Close the Window, and you should have a download ready. Attempting to play the game should automatically trigger the download.
How to switch back on Steam:
To switch back and opt out of all betas, simply repeat the Steps above, but select "None - Opt out of all Betas" from the drop down list in Step 3.
To access the Beta Branch on GOG Galaxy, please follow these steps:
1) Proceed to the game card for your game in the GOG Galaxy client.
2) Proceed to the game's settings via the "More" option, and select "Settings"
3) Select "ON" from the drop down menu in the "Beta Channels" option on the right hand side.
4) A "Channel" option should now be visible. And in the Dropdown should be the option for "Beta"
5) After selecting this option, GOG Galaxy should automatically start your download of the beta branch.
How to Switch back on GOG Galaxy:
To switch back, simply repeat the steps above, but choose "default" in the Channel Drop down options.