There are a number of ways, but one of the easiest, and one that works on all modern versions of Windows is this:
In details: hold down the Windows key, press and release the “R” key, let go of the Windows key.
The “Run box” will appear:
Type “winver” (without the quotes) and click “OK”.
You’ll be shown a box that tells you which Windows you have (Windows XP, Windows Vista, Windows 8, Windows 10), and, in smaller letters the version and edition – version 1610 of Windows 10 Pro, for example. Click “OK” to make the box go away.