Leaking Gas help needed!!

I have a 96 GT and I went to fill up today and just as I got it filled up it started seriously leaking gas so I looked under and it seems to be coming from the place where the filler neck meets the tank the o -ring seems to be ruined. My question is how do you change that 0-ring do you have to drop the tank or what? Also is this a difficult fix? Thanks.
 
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