How do I adjust the headrest?

When using the bottom two sets of slots, position headrest level with top of seat. For top four harness slots, place headrest about level with jaw - one inch abov the shoulders. Adjust by pressing lever on headrest and sliding it up or down until you hear an audible 'click'. (Page 30-31)

From the official Diono Radian 3 R manual, page 30

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