Coachella and Stagecoach refund policies state that all sales are final. This information can be found in their FAQs on their website.
If you would like to sell your ticket through the fan-to-fan exchange you are welcome to do so! Those links can be found below:
Coachella Weekend 1: https://coachella.lyte.com/94087
Coachella Weekend 2: https://coachella.lyte.com/94088