Back up your iPhone 11
Check the iOS version you want to install
Make sure Apple is still signing that iOS version
Download the correct IPSW file for iPhone 11
Install the latest version of iTunes on Windows or use Finder on Mac
Connect your iPhone 11 to your computer with a USB cable
Put your iPhone 11 into Recovery Mode
Open Finder or iTunes
Select your iPhone when it appears
Hold Option on Mac or Shift on Windows and click Restore iPhone
Choose the downloaded IPSW file
Confirm the restore and wait for the process to finish
Set up your iPhone and restore your backup if needed