If you have an iPhone running on iOS 12 and you need to turn off Find My iPhone without the password, it can be a tricky situation. However, there are a few things you can try to resolve this issue:

1. Reset Apple ID Password: Try resetting your Apple ID password through the Apple website. This might allow you to turn off Find My iPhone without needing the old password.
2. Use Recovery Mode: You can try putting your iPhone in recovery mode and restoring it through iTunes. This can sometimes bypass the need for the Find My iPhone password.
3. Contact Apple Support: If the above methods don't work, reaching out to Apple Support might be your best option. They can guide you on how to proceed without the password.

It's important to note that these methods may not always work and can vary based on individual circumstances. Make sure to have all necessary information and proof of ownership before attempting to turn off Find My iPhone without the password.