Hi, Jacob!
Build earned at an event is spent on the section on the back of your character card--it's called Skills Learned/Unlearned. When you learn a new skill (provided you have enough build to learn it), you write the skill there with the build cost and the card number of the player who taught it to your character.
Another way to spend the build you've earned, considering we're still in the midst of our rules rewrite, is to follow the instructions here and submit a rewrite by October 14th:
docs.google.com/forms/d/e/1FAIpQLSeZcBkw...L8go479-RGA/viewform
If you need to only change a couple of skills rather than rewriting your whole card, you're encouraged to instead submit a Card Problem Report in the Card System--and if you need help figuring out how to do that, let me know in a comment or message either here or on Facebook.
Cheers,
Jeff
