+1 on letting UA know that you will not be flying. The question is when to do so, since dropping the segment will most likely trigger a refare.

If you cancel before taking the first flights, you can confirm the refare by pricing out the new itinerary and note the difference.

I would normally recommend cancel the DTW flight after taking the first flights, that way the refare will only impact the last flight to HNL. However, depending on the timing and advance purchase fare rules, that might increase your fare compare to cancelling earlier.

Then again, it might not hurt to ask an agent whether you can just cancel the segment you are not planning to fly without refaring. Unlikely, but they have been more willing to do favors and waivers in my personal experience.