I'm going to assume since you are asking this, that you have a pen type mod. Most fill from bottom, unscrew from battery, turn the
tank upside down, and unscrew base from
tank. Fill liquid slowly, and most have an inner chimney, tube looking piece. Don't get liquid in it, and don't fill above it.
Best option though is to go to the shop you bought it from and have them show you. If that's not an option look up you device or tank on youtube.