Edwards & Co OSCAR MX HMV501 Instruction Manual page 20

SHOULDER STRAP HEIGHT
There are three height slot positions for the shoulder straps. As your
child grows, you will need to move the straps according to your
child's height. The correct height is where the straps are about level
or just above your child's shoulders. The picture on the next page
demonstrates the correct installation.
To adjust the height - find the harness loops inside behind the seat.
Remove and insert the hooks into the loops.