how can reduce swelling on knuckle after training ?

Wear handwraps.

put ice on it.

I've always been told to put your hand in a bucket of ice water for about 5-10 minutes at a time. Always works for me.

Ice it, and don't do any punching for a few days if it's really sore.

Always wrap your hands, don't hit bags without gloves, and put some sponge or other padding over your knuckles before wrapping your hands for extra protection.