is currently impossible through official Apple channels. Apple stopped signing iOS 13.7 on , shortly after the release of iOS 14.
Because Apple no longer authenticates this software, you cannot install it even if you have the original firmware file. Why You Can't Download iOS 13.7 Download iOS 13.7 for iPhone SE
Downloading iOS 13.7 for an Go to product viewer dialog for this item. is currently impossible through official Apple channels