Q: iPhone 4S downgrade

HI

I upgraded my iPhone 4S to iOS 9.3.5

Since the day I upgraded I am regretting it. The device have started behaving like a retard.

It automatically restarts after every 30-40 mins. Its like anytime you see the device it is off. Any updates to any app will restart the device. Using any app on the device, even making or receiving calls is a nightmare.

I am planning to buy iPhone 7 now. Is there a way I can get my iPhone 4s back to iOS7 or earlier?

Atleast I can clean and give it to somebody who can use it then?

Thanks