A bicycle helmet needs to be comfortably in contact with the head, level, and stable enough to resist stay in place in a crash. It should be as low on the head as possible to maximize side coverage, and held level on the head with the strap comfortably snug. https://helmets.org/fit.htm

How to fit a helmet:

Adjust the fit pads or ring

Adjust the straps

Helmet Fit Checklist: https://helmets.org/checklist.htm