Those who do not take advantage of the offer will have to pay for it, with the home edition costing $120 (£91) and the professional edition $200 (£151).
Eligible devices include tablets and PCs running Windows 7 or newer.
Microsoft has been criticised for heavy-handed tactics in promoting the upgrade.
In May, the software giant was accused of a “nasty trick” when it changed the pop-up box encouraging users to upgrade so that clicking to shut it down meant the upgrade was scheduled rather than rejected.
A Californian woman successfully sued the firm after she claimed that an automatic Windows 10 update left her computer unusable.
In June, Microsoft said that is was going to offer “clearer options” for users considering the upgrade.
To date, around 300 million devices worldwide have upgraded to, or are using, Windows 10.