Diagnosis
Dust, pollen, and water spots are the main culprits for blurry video, and using the wrong cleaner can make a haze or damage coatings.
How to Fix It
Power off the camera by unplugging it. Use a can of compressed air first to blow away loose dust. Then, take a microfiber cloth (like for glasses or camera lenses) slightly dampened with distilled water—not tap water—and gently wipe the main camera lens and the small ambient light sensor window next to it. Dry immediately with a clean, dry part of the cloth.
If That Didn't Work
For stubborn tree sap or bug residue, a drop of isopropyl alcohol (90% or higher) on the microfiber cloth can work. Rub very gently. Avoid any 'all-purpose' cleaners, window cleaner, or paper towels, which will scratch or haze the lens.
Heads Up
The lens has a hydrophobic coating to shed water. Harsh chemicals or abrasive wipes will strip this off, making future water spots worse. Also, don't spray anything directly onto the camera; always apply cleaner to the cloth first to avoid liquid getting into the seams.