Bests & Worsts Reviews from Amazon

How do I give negative feedback? I heard such great things about this toothbrush so bought it for my 16 month old son. During his first use, he kept on scratching his tongue and making funny faces. I didn't realize until I looked at his toothbrush that all the bristles came out and some were roaming around his mouth, some were stuck between his teeth, and the other half were half way out of his toothbrush- see attached photo. Now- how do I get a refund for this?