Q: How do I update my new iPhone 6 phone with 8.3with my old update 8.4

Your question is not very clear.

Please explain in detail what it is you want to do.

I believe you are asking how you can restore an iPhone 6 with iOS 8.3, with a backup of a device that had iOS 8.4 on it? Is that correct? If so, then you need to connect the iPhone 6 to iTunes, update it to iOS 8.4, and then restore the device to the old phone backup. Will work like a charm after you complete the update.

I Agree with Phil. Your question doesn't make sense. But I am taking a guess. If you are trying to restore the backup from your old phone that was running iOS 8.4, to the new iPhone running iOS 8.3, you have to update the new phone to iOS 8.4 first, then restore the backup.

You can update the phone in Settings>General>Software Update. Depending on where your backup is, if I is in iCloud, you will have to erase the device after updating and then restore the backup. If you backup with iTunes, you have to connect the phone to your computer's iTunes and restore the backup there.

This is might be helpful.

Transfer content from an iPhone, iPad, or iPod touch to a new device - Apple Support