Please can you tell me how I cancel a subscription that is on my iphone? The iphone has broken and have been told to cancel it on itunes???

I need to cancel a weekly subscription that is through an app on my iphone - the iphone isn't working and has been sent away so they have told me to cancel the subscription through itunes?? I have tried allsorts and really don't know how I can access my itunes account to cancel subscriptions. Please help as I can't afford for this money to keep being taken off me.