Downgrade iPhone 4S from iOS 9.2 to iOS 7.1.2

Downgrade iPhone 4S from iOS 9.2 to iOS 7.1.2

My iPhone 4S has some kind of short circuit.

The battery drains within half a day.

I cannot switch it off (stand-alone).

I can only switch it off when it's connected to a power supply.

When it's off and I unplug the power supply then it's automatically activated.

That's the reason why I want to go back to my iPhone 4 with iOS 7.1.2.

When I want to restore my iPhone 4 from the backup of the iPhone 4S

I obtain the error message that the backup cannot be restored

because the software on the iPhone 4 is not up-to-date.

That's the reason that I need to downgrade from iOS 9.2 to iOS 7.1.2 on the iPhone 4S.

Can anyone tell me what I have to do

to downgrade from iOS 9.2 to iOS 7.1.2 on an iPhone 4S?

Any help is appreciated!