"To adjust the seat" means to change the position or settings of a chair, car seat, or similar object so that it is more comfortable or suitable for the person using it. Full definition
Adjust the seat height so that the handles are level with your chest. (bodybuilding-wizard.com)
You can easily adjust the seat position from the highest for playtime to the lowest for sleeping time. (followmum.com)
Starting Position: Simply begin by adjusting the seat so that your knee lines up with the pivot point identified on the machine. (best-leg-exercises.com)