Hello, I recently submitted my SIR to UC Berkeley to begin undergrad there starting in the fall. However I just remembered that on my application I put that I planned to take the AP Euro exam this year, and in a side comment explained that I was self studying (because it isnt offered at my school). I did self study a good amount of the material first semester, but then I got very busy during the spring when baseball season started. Because of this I missed the deadline for paying/registering for AP exams at the other school i was supposed to take it. 
It’s too late now to take the exam, so what should I do? Will my admission be revoked when I tell them know?
This is less than nothing. You’re fine. You don’t have to say anything about any AP exams. You could purposefully skip every one and you’d face zero ramifications.
UCs are sort of sticklers for doing what you reported as far as taking classes, but this is likely a non issue.
I have no specific knowledge of UCB’s policies regarding your issue. However, if you’re concerned, you could always provide an e-mail that indicates why you’ll now complete the self-study during the Summer months . . . and, of course, do so.