Infant and child car seats often come with several shoulder heights so the car seat can be adjusted as your child grows. The shoulder height of the harness is determined by slots in the back of the car seat and adjusting the shoulder height means having to rethread the harness straps through the different slots. It is extremely important to make sure you rethread the 5-point harness properly so you're assured of your child's safety while traveling in the car.
Sit your child in the car seat and determine which slots you need to use. For rear facing car seats, you usually use the slots just below the child's shoulders; for front facing car seats, it's usually the slots just above the child's shoulders. Make sure you follow the manufacturer's guidelines for your car seat.
Have your child get out of the car seat and flip the car seat over so it is face down. Unhook the loops on the straps from both sides of the metal connector. Pull the straps out through the front of the car seat.
Hold the harness straps out straight and flatten them so they are not twisted. Make sure the straps remain untwisted during the rethreading process.
Slide the ends of the harness straps through the slots you measured when your child was in the seat. You may have to press the loops flat to put them through the slots. Be sure you use the same level slots on both sides of the car seat.
Push the loops at the ends of the harness straps over the metal fastener on the back of the car seat.
Install the car seat in the car and have your child sit in it again. Place the shoulder straps over the child, and clip the buckles together. Pull on the adjustment strap under the crotch strap to make sure the harness fits your child snugly.