how do u cancel the purchase after winning the bid on ebay. i won a bid for a product but i do not want to buy it now. it just turned out to be a risky buy later so i thought i should not go ahead and pay.

Just dont pay. Every member can leave three items they bought. After third one your account will not be able to bid for anything. 
Make sure you email your seller, some time they mark it as send and there will be no negative points on your account. 
One time I bid and won non working blackberry for 45 bucks, i was stupid for that , I thought its working phone. I emailed seller and he marked it as send and even he gave me 5 star as good buyer. 
so email seller first. 

 thanx dude!!

I think UBID has some option to re-tract the bid, not sure in Ebay, just write to the seller that you bid on wrong item
Most likely they will honor your request.

email the seller. most of the time, they send you cancellation request so they don't have to pay the closing fees and you dont have to pay for your bidding. simply accept it and it would be closed without any negative stuff