SharmaDTS :
What is ISO and if it can be done without formating then is it sensible & a good idea to format and then install Windows 10
An ISO file is simply a recreation of a DVD in a single file.
All that means is you can use that to create an installation DVD or USB, and do a full install.
Either way works: Windows Update, or full wipe and reinstall.
(according to current info, anyway)
Bear in mind, this is a new OS, so there may be some issues on the initial rollout. Don't be too hasty to upgrade on day one.