Downgrade rights to Windows 7 Professional would have to be supplied by you the OEM System Builder.

Downgrade rights are normally only available to name brand systems such as DELL, HP, LENOVO, Acer that come preinstalled with Windows 10 Pro at the factory.

If this is not an option available to you, you will have to purchase the Windows 7 Professional license.

You can find OEM System Builder software from dozens of online merchants. The current price for OEM Windows 7 Professional at Newegg, for example, is $140. When I checked a few minutes ago, Amazon was offering OEM Windows 7 Professional packages from multiple sellersat prices ranging from $101 to $150. When I checked just now, a package specifically intended for refurbished PCs cost only $50 for a 64-bit copy.

There are no technical limitations to prevent you from using OEM software on your own PC, although this software will work only for a clean installation and not for an upgrade. In the past, Microsoft has been remarkably inconsistent in its advice to customers about whether this practice is allowed. (See "Is it OK to use OEM Windows on your own PC? Don't ask Microsoft.")

Important: the information on this page is intended for system builders and the downgrade rights that apply to their customers. If you are a customer of a direct OEM, please contact your OEM for more information about downgrade rights. If you are a direct OEM, please contact your Microsoft Account Manager.

Downgrade and down-edition rights are an end-user right that Microsoft offers to customers for certain OEM products which meet the technical requirements for a Windows software downgrade. Downgrade rights are documented in the Microsoft Software License Terms that customers accept upon first running Windows and Windows Server software. Thanks to downgrade rights, end users who have acquired a later version of the software (such as Windows 10 Pro or Windows Server 2012 R2 Standard) can use an earlier version of the software until they are ready to migrate to a later version technology.

If a product includes downgrade rights, the license terms for that product will indicate which earlier versions of the software may be used.

Downgrade rights will vary depending on if the software was acquired via volume licensing, OEMs, or full packaged product (FPP).

The following OEM versions of Windows software are eligible for downgrade rights.

Note downgrade rights are only available as long as Microsoft provides support for that earlier version, as outlined in the Windows lifecycle fact sheet.

Steps for an end user to downgrade Windows software

To downgrade eligible Windows software, end users must:

• Purchase a PC preinstalled with Windows software.
• Accept the Microsoft Software License Terms.
• Perform the downgrade process to the eligible downgrade product using the media/key from a genuine, previously licensed OEM or retail product.

The downgrade process

Follow these steps to downgrade to an earlier version of Windows software:

• Use genuine Windows media and a corresponding product key for the version of Windows that is eligible for downgrade. The media/key should come from a previously licensed product from the OEM or retail channel.
• End users who are licensed separately through Microsoft Volume Licensing (VL) may provide their VL media and key to a system builder to facilitate the downgrade on their own systems.
• Insert the downgrade-eligible version of Windows media and follow the installation instructions.
• Enter the product key.
• If the software was previously activated, it cannot be activated online. In this case, the appropriate local Activation Support phone number will be displayed. Call the number and explain the circumstances. When it is determined that the end user has an eligible Windows license, the customer service representative will provide a single-use activation code to activate the software. Please note that Microsoft does not provide a full product key in this scenario.
• Activate the software.

For scenarios involving large volume downgrades, end users are encouraged to consider VL which provides the benefits of reimaging rights and volume activation, which can make the process of downgrading much easier.

Authorizing a third party to exercise downgrade rights for an end user

Because downgrade rights apply to end users, they are not designed for third-party facilitation, which has many complications. Also, such facilitation is not suitable for carrying out on a large scale.