Although I've often bought from eBay, I've never sold anything on it, so I'm no expert. However reading through this suggests that you'll have to use "Sell Similar" (and not "Relist") to offer the item again with a correct listing - and then cancel the first one:...
I think you will find you cannot edit the listing from a buy it now price to an auction starting price though. Other aspects of the listing can be changed but not that