Q: iCloud backup is iOS 8.3, new iPhone has 8.4

Hello! Really need help with this fast.

I did an iCloud backup of my iPhone 6 Plus iOS 8.3 yesterday because I needed to repair one button on my iPhone.

Instead they switched the whole iPhone out so today I got a brand new iPhone 6 Plus with iOS 8.4 installed.

Now when I'm trying to set it up from my iCloud backup I did yesterday it says that the iPhone need to be iOS 8.3.

Tried downgrade it with no luck with a ipsw file..

Please help me. I'm in need of my phone with the exact copy of my old one.

Thanks