Apple has charged me after 3 months when I subscribed for 6 months of trial for Apple Music.

I subscribed to the 6 months trial on apple music via Uni days subscription. When I was registering apple stated that I would get an email before the end of three months and then I have to renew my trial to enjoy 6 months of Apple Music trial. But suddenly I got a reciept that I was billed after three months without any prior notification. What can I do regarding this. When I contacted Uni Days they said to contact Apple music. I found no way to contact them to report this issue. What can I do regarding this?