Here is the complete guide to update and download your iPhone/iPad to lower or higher versions of iOS, while iOS firmware version unsigned. iOS 11, iOS 12, 13 and iOS 14.
update your iPhone/ iPad which running iOS version lower iOS 14 (And below also iOS 13.5 etc.), iOS 14.1, iOS 14.2 to iOS 14.3.
Complete iOS 14.3 upgrade guide without having SHSH