Windows 10

[Pudcat]

What is the cheapest way to download Windows 1

[CrapAtCryptics]

Get a copy from a reputable buyer on Ebay, I recommend the Pro version should be about £5 for the key and will allow any upgrade from 7 or 8

[Buenchico]

Make sure that you know which version of Windows 7 or Windows 8 you're currently using. (i.e. 'Home', 'Pro', etc). Find out the licence key for it. (You can do that by downloading, installing and running Belarc Advisor: https://www.belarc.com/products_belarc_advisor )

Go to the Microsoft site and look for the download to upgrade from Windows 7/8 to Windows 10, ensuring that it's the same version as the one you're currently using.

Install the upgrade. When you're asked for the licence key for Windows 10, use your old one for Windows 7 or 8. (It'll work just fine).

Job done - for free!

[Gimont]

@ CrapAtCryptics
All about Windows update are free?

[WilmaAumille]

Download is free. Actually, it is free to use it after install but most of the features are locked without activation. As for Windows 10 download, you can use the official microsoft link, which downloads a file called Media Creation Tool. It is the one for download and installation.
If it does not work, then you can use third-party tools like Rufus or HeiDoc Microsoft Windows and Office ISO Download Tool.

Step by step tutorial: https://www.uubyte.com/download-windows-10-iso-image-free.html