Given that Microsoft are wanting all windows users weather they want it or not very little options to avoid the inevitable!
Free option to update to windows 10 from Vista/windows 8/8.1 ends July 2016.
Depending on your download speed, allow one and half hours to do the initial installation.
Updates to bring it to current, well how long is a piece of string!
Brian
[Watch out for current applications being deleted on installation of new operating system]