You cannot fully uninstall an iPhone update once it is installed
You can downgrade only if Apple is still signing the older iOS version
Back up your iPhone before making any changes
Check whether the older iOS version is still signed by Apple
Download the correct IPSW file for your iPhone model
Put your iPhone into Recovery Mode
Connect your iPhone to a Mac or PC
Open Finder on Mac or iTunes on Windows or older macOS
Select your iPhone
Hold Option on Mac or Shift on Windows and click Restore iPhone
Choose the downloaded IPSW file
Wait for the restore process to finish
Set up your iPhone again after the restore
Restore your backup if needed
If Apple is no longer signing the older version, you cannot downgrade
If you only want to remove a beta update, unenroll from the beta program and restore the latest public iOS version