I have windows 8 1 and want to go back to windows 7 and

My pc is laptop model no c551-a5314 ser no9d064714q-dc19v-3.42a

I understand your perspective completely. Windows 8 has been good for a massive amount of confusion and dissatisfaction. However there's good news. You can download a small free program called Classic Shell and it will make Windows 8 look and feel like Windows 7. On the other hand taking a computer that was built for Windows 8 and trying to downgrade it to Windows 7 is expensive and frought with peril. Please give Classic Shell a try; it's made most folks (myself included) content with Windows 8. You can download it from classicshell.net. I'm younger than you, but only by 11 years.`
Good luck.

First of all, you'll need to buy a retail version of Windows 7 (or, at least, the license key). That's not cheap. Then you need all Windows 7 drivers for your laptop from the makers site. If they don't have those, it's a no-go.
Then you need a safe place to store your pics while installing Windows 7. That's a big enough USB-stick, an external hard disk or Dropbox.

It is much cheaper and much easier to install Classic Shell (free download) and make Windows 8.1 lookso much like Windows 7 that you hardly see the difference.

Kees