How to Revert Back the old OS of my PC?

As a Windows 10 user I was very happy. But, after I see the peculiar ads pop-up now I want to revert my computer back to Windows 7 OS, but the process to do it is not well-known to me. I have searched several times regarding this topic. I guess I need to take help from the Acer Laptop Services Center, whether I am going right or not?