Got a D+ in AP Calc AB first semester of senior year

So I got a 68.7 in AP Calc AB first semester of senior year and my other grades were still pretty good, its just that D that makes my grades look ugly. This was the only D I got in my life and I don’t know how I made it get this low and my only excuse was the class was hard. I know this could affect my chances for UC’s as they don’t want Ds or Fs, but like what should I do? Should I email them? if so how do I do that?

Here is the info:
Freshmen: If you change schools, add or drop a course, or fail to earn a C or better in a course after you submit your application, you must notify the UC Application Center by email or postal mail. Your correspondence must include your name, UC Application ID number and your signature (if you mail a letter), and will be available to all the campuses to which you applied.

If you receive an offer of admission, be sure to confirm with the campus admission office that they are aware of the deficient grade or schedule changes. Although you must notify the campus of these changes, we cannot guarantee that a campus can go back to review an application after correspondence has been received. Make sure to save a copy of your correspondence just in case.

Email: docs@applyUCsupport.net

Postal mail:

UC Application Center
P.O. Box 1432
Bakersfield, CA 93302