Your question has a two part answer. From a licensing standpoint, your edition of Windows (e.g. Home Basic, Home Premium, Pro, Enterprise, etc.) has to be eligible for downgrade. Microsoft provides information about this on their OEM Partner
The other part is having your original installation media and license key.
The licensing would be taken care of by your OEM (Lenovo) so depending on what version of Windows 10 was installed on the computer you bought you may already have Windows 7 downgrade rights. If you bought a ThinkCentre, ThinkStation, ThinkPad, Idea Tablet, IdeaPad, or IdeaCentre with Windows 10 Pro from Lenovo you have the right to downgrade your computer to Windows 8.1 Pro or Windows 7 Pro. If you need the Windows 8.1 Pro or Windows 7 Pro Recovery DVD you may be able to purchase it from Lenovo.1
Please let me know if you have any other questions.
Edited by packetanalyzer, 21 October 2015 - 11:52 AM.