Go to this website[0], type in your zipcode, and write a letter or email to the address provided.
It may also help reading this[2]
[0] https://www.congress.gov/members/find-your-member
[1] https://www.house.gov/representatives/find-your-representati...
The best way to invest "against" the F-35 is to put money in companies like Embraer and Saab, which have put up a strong fight on the export market.
FWIW, the Super Tucano and Gripen are more "morally acceptable" in that they are not power-gap level planes. The F-35 was basically designed from the ground-up to molest denied airspace and violate every rule of diplomatic exchange. The Super Tucano is useless outside COIN operations, and the Gripen is decidedly a defensive fighter and not a strike fighter.