Before You Start
Please make sure you know what you’re doing and back up your saves, you’ve been warned!
Before you do opt in, just be aware that sometimes a build in the PTE can be broken or may not be compatible with your current save files (either entirely or partially).
Opting In
Go to Narita Boy in your Steam library, right-click and select ‘Properties’
Now select the BETAS tab, click on the drop-down box and select public_test_branch.
Once selected, the game's title in the library will have public_test_branch after it, this means it has worked.
Troubleshooting
But it's not working
It's possible that the download hasn't started, head to downloads and force the download to start. If you want, turn on "Always keep this game up to date".
Still not working
In Narita Boy/Properties click on the LOCAL FILES tab.
Choose the option VERIFY INTEGRITY OF GAME CACHE.
This should solve the issue.
The build is too buggy, I want the old build backNo problem, first make sure you've helped out the Devs and reported any bugs you've seen, here.
Ok now that's done:
1. Go to Narita Boy in your game library, click and select Properties
2. Now select BETAS tab, click on the drop-down box and select NONE - Opt-out of all beta programs
3. You're back to normal!
Source: https://steamcommunity.com/sharedfiles/filedetails/?id=2463021439