Graco 8J00MTX - Nautilus 3- In 1 Toddler Car Seat Owner's Manual page 11

6. Squeeze height adjust button and push
head support into slots on the back support
as shown. Head support will click into the back
support.
You may need to unfasten the hook and loop
attachment tape on both sides at the top of
the back support to allow for easier access to
the slots.
Always use child restraint with head support
installed and properly adjusted.
CLICK!
11