Use your steam key to redeem your BBU copy
Right-click the game file, then properties, and then Betas

Password: BBUTrollBossFightBuild (please do not share this password, if anyone is found to be sharing it they will be permanently banned from Patreon and any future builds)
Select the Troll beta build and wait for the download

Enjoy!
This fight is still early days and will receive a lot of balance changes throughout. The red squares are "stun flowers" and will stun the boss for a short amount of time (and eventually Billie also if you are not careful). You will learn this mechanic in the level before the fight.
Let me know in the comments below if you still need a key. Your old key will still work!