Ask them politely to lower it. If they don’t, you have two options:
A. Leave your seat to go find a security guard, missing part of the show in the process and potentially having this person fuck with you for the rest of the show in retaliation.
B. Rip that shit straight out of their hands and toss it to the lower arena. They’ll have no idea who did it because their view was blocked by their umbrella. Don’t have to miss a second of the show.