If you are still on Windows 7 or 8, you can upgrade to Windows 10 for free. Windows 7 and 8 will upgrade to 10 and Windows 10 will upgrade to 11, if your computer meets Windows 11’s stringent system requirements, which include TPM 2.0 support, at least 4GB of RAM and at least 64GB of storage space (you can bypass Windows 11’s TPM and RAM requirements but we don't recommend it). If you already have a prior version of Windows installed on the computer and you just want a newer version of the OS on the same hardware, you can likely upgrade it for free (obviously this method doesn’t help if you are building a new PC).
Upgrade from a Prior Windows Version: Free
