iPhone stuck on "Preparing Update"

Try the following:

1) Restart your iPhone, turn it back on, and retry the update.

2) Check your iPhone storage and see if it is low (see here for storage optimization: https://support.apple.com/en-us/HT201656, also delete any unneeded apps, downloaded movies, shows, or music if needed).

3) Backup your phone, reset iPhone to factory settings, and then set it back up with the iCloud backup + update the phone during the setup process.

See if any of these work for you.