It'd be weird for an ERP system to go through an upgrade without consultants from the vendor (Epicor) or a 3rd party systems integrator to handle the hands-on-keyboard part of the work. In a normal situation I'd expect your role to be that of a smart phone directory, able to get hold of the right people when the Epicor techs need to talk to someone.

It's also strange that you haven't been involved in calls with the vendor or any of the planning meetings that should have been going on over the last few months. See if you can find out what the actual plan for the upgrade is.

Maybe there isn't one. This could be a seat of the pants operation and you could be about to walk into a total clusterfuck. That said, DO NOT LEAVE! There are no repercussions for you, the intern. You could walk over and pull the plug out mid install and it'd still be your bosses fault for leaving an intern in full charge of anything more important than the spare coffee maker. You have a ringside seat to a fantastic learning opportunity here - you cannot lose. Enjoy the ride, keep a diary, use your head, do your best. Best case: you're a savior (enjoy the $5 Arbees gift voucher). Worst case: the upgrade is delayed by a week, your lazy boss gets canned, and everyone else views you with a mixture of sympathy and respect and you get onto a new team.