Remove the visor from the helmet to reach inaccessible places (no tools required).
Mix up a washing solution made of warm water and a mild soap
Soak a paper or microfiber towel in the washing solution and place it over areas where dried insects are stuck, letting it soak for several minutes. The dirt gets easier to remove.
Gently rub entire outer surface with the washing solution and paper or microfiber towel or your fingertips; do not scrub helmet shell using pressure with a towel (this causes scratches)
Use moist cotton swabs to clean inside the vents
Wipe away any soapy residue with a moist microfiber towel
Carefully rinse the helmet, making sure that water does not enter into the helmet.
If your helmet has any electronic parts such as Bluetooth device, its important to be very cautious with the water.
Dry the shell with a dry microfiber towel
Once dry, you can lubricate visor sealing and control/flip mechanics with silicone oil.
Never clean your helmet with aggressive chemicals such as gasoline, solvents and aggressive or abrasive agents.