I bought one Lenovo laptop built in 8.1 OS. How can i downgrade to 8?

You have to buy the operating system disc for Windows 8 and remember to download the right drivers as track pad and software wouldn't work

If it came with 8.1 then you would need to purchase a windows 8 disk and install it yourself, you can't downgrade

The only stupidity I found in 8.1 is the cloud storage and it is complicated to get rid of it, but that would be the suggested fix of 8.1

I already use classicshell.net and changed every full-page win8 app to a freeware using a Window.
Windows 8 is a failure, and 8.1 was the fix, and the muckup is in the cloud storage that slows down the start-up and is integrated into the operating system.

For all intents and purposes, although you can spend $90 on Windows 8 and collect the drivers, wipe the HDD, and start again, I would aim at the fixes instead.
There are complex ways to get rid of skydrive, and eventually, I will do it.