My Taz weighed 21 1/2 lbs at his heaviest. Up until I moved in with the BF all of the vets I saw weren't too concerned about it. Because he was still active and could jump onto windowsills and tables with no problem.
But the vet here said she'd like to see him lose a couple pounds. I put him on a diet food purchased at the vets office. But you can find diet food in the pet food stores too.
Well, it doesn't work quickly, that's for sure. And then Taz got sick and lost too much weight! He has maintained his weight for the last few years at 12 1/2 lbs. Which is a bit skinny for his size, but now I can't seem to get weight back on him! LOL

But I just suggest a diet food (someone else can suggest what brand and what to look for in the ingredients, I'm not good at that either) and lots of play time to help Star get more exercise.

Good luck!