Download iOS 13 firmware from following links and install it.
iPhone 6s:Download
iPhone 6s Plus:Download
iPhone SE:Download
iPhone 7:Download
iPhone 7 Plus:Download
iPhone 8:Download
iPhone 8 Plus:Download
iPhone X:Download
iPhone XR:Download
iPhone XS:Download
iPhone XS Max:Download
+ Windows 10 users: The current version of iTunes doesn’t support iOS 13 IPSW files yet, but you can install iOS 13 using a command line tool on Windows 10. Check out the link below:
Mac users: You can either install the macOS Catalina Beta or install Xcode Beta 11 on your Mac to be able to install iOS 13 on your compatible devices. For more details, check the link below: