How do you move weight on semi trailer?

The question's a bit vague. If you're referring to distributing weight once you're loaded, it depends on what your trailer has for axles. If you have fixed tandems or a fixed spread axle, you would have to move the load itself. If you have sliding tandems, you can move them forward to shift weight from the drive axles to the trailer axles, or you can move them back to shift weight from the trailer axles to the drive axles. If you have a sliding fifth wheel, you can move that forward to shift weight from the drive axles to the steer axle, or you can move it back to shift weight from the steer axle to the drive axles.