if it's the bottom leaves that are yellow....too much rain has caused that..............sometimes tiny bugs cause the same thing.
after the sun goes down mix 1 teasp of liquid hand soap with 1 qt water and spray the plant tomatoes & leaves and under the plant..........can repeat if needed......if you decide to cut off those yellow leaves on the bottom....use sharp scizzors and have a plastic store bag ready to drop it in tie it up and trash can it.......