2 offers

If you renege, then it will burn a bridge regardless of how nice your email is. It basically comes down to what you want to do and your level of comfort with reneging. You clearly would prefer the other offer or you wouldn't have interviewed after accepting something and you wouldn't be asking for advice on how to renege. If you want to do that instead, just go do it and tell JPM that you're going a different route. They will be pissed / annoyed, but you aren't the first person to renege.

Just call the recruiter you worked with ASAP and tell them your career aspirations have changed and while you appreciated the opportunity, you want to head in another direction and apologize for the inconvenience. Its really not that big of a deal but yes, there is potential to burn the bridge. Just know you'll be better off for it, at the end of the day it is just for an intern position.

Be respectful but do what's best for you. The recruiter will be ticked off but it's not like you'll be blacklisted from JPM for life.