You have paid for a License when you purchased your Lenovo with Windows 10 preinstalled and that License is for Windows 10.
Your Windows 10 License does not entitle you to use a different operating system with the Windows 10 product key/license.
If you had Windows 10 Pro preinstalled you could have downgraded to 8.1 Pro.
As you haven’t, you have to buy Windows 8/8.1.
Microsoft no longer sells Windows 8/8.1.
And support has ended for Windows 8 in Jan, 2016.