I'm guessing that you have a Warrior tier (your post is very vague). We can best help you if you tell us what Tier and what specific hilt you have.
For a Warrior saber, simply unscrew the pommel at the bottom of the saber (opposite of where the blade goes). Then you would pull the battery pack out of the saber, remove the battery (it can be very tight), and put it in the charger. Do note that the included charger with the Warrior tier is dirt slow and that it can take 12-24 hours to fully charge; I would recommend buying a better 18650 charger if you can.
If you have a Champion or Hero, then you would have a recharge port / kill key combo. You do not need to open the saber at all. Simply plug the included charger into the port that you pull the kill key out of.