How to slim down my legs?

Look up lower body workouts on youtube. For slimmer legs, if you aren't carrying extra fat, you need to work the muscles. More muscle will make your legs more tight/compact and firm and thus appear smaller. Do exercises without weights but plenty of reps. Running also helps tone the lower body.

If you want slimmer thighs you need to lose weight. No specific exercise targeting thighs will help.