How should i weight train to increase explossive speed and power for boxing?

Plyometrics: For explosive speed and power, i would recommend plyometrics (you can google that), which involves rapid bursts of activity such as jumping exercises and throwing medicine balls.

Don't bulk up!: If your goal is for explosive speed, then you want to build up your fast twitch muscles and also throw your punches without tightening your arm muscles as you throw the punch until the point of impact. Compare your speed when you throw your arm out without power and then when you are throwing a punch as hard as possible. The relaxed punch is a lot faster. Speed bag and footwork with a good coach