I was planning on transfering to UCI for fall 2016 in software engineering, but I recently received a D in calculus this past semester. Any advice on what I should be doing now?
retake the course
you are required to notify UC of any grade less than a C. See http://admission.universityofcalifornia.edu/how-to-apply/after-you-apply/ I wouldn’t suggest skipping this; they will see your transcript this summer and will find out you didn’t notify them. Maybe they won’t care if you pass it when retaking, maybe they will.
Figure out why you are not learning this material. I have some comments & suggestions in the thread http://talk.collegeconfidential.com/discussion/comment/19090340/#Comment_19090340
If you have applied to UCI via TAG then talk to your counselor or contact UCI to see if this voids the contract.