Once you have the Leap_10.3.3.1435_Signed_Autoloader.exe file, follow these steps precisely.
Official download links for the BlackBerry Leap OS 10.3.3 autoloader from BlackBerry's own servers are largely defunct following the end of legacy services on January 4, 2022. However, you can still find the required files through community archives and third-party mirrors. Internet Archive - BlackBerry 10 Autoloaders
Note: You must copy the entire URL into your browser.