Q: How to uninstall a new ios update

You don't.

iOS updates cannot be undone.

If you are having issues with an update try the normal troubleshooting steps of:

Restarting, Resetting all Settings, Restoring, and if all else fails Restoring without a backup.

Restart your iPhone, iPad, or iPod touch - Apple Support

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

Use iTunes on your Mac or PC to restore your iPhone, iPad, or iPod to factory settings - Apple Support

Since this is unrelated to Windows or Software Updates for Windows software (the forum you posted in), I will request this be moved to a more suitable forum.