You will be able to download the update through Windows Update or you can download .ISO media and use it do an in place upgrade. For many persons, using ISO media will be the preferred method since you will have a backup copy for reinstall purposes if needed. Also for persons with limited bandwidth, this will be an excellent way avoid the need to download the update more than once if you have multiple computers. So lets take a look at upgrading to Windows 10.
Please be aware that you cannot use the free upgrade offer to perform a clean install on first attempt
You must first upgrade from the qualifying version of Windows 7 or Windows 8/8.1 (whether you do it through Windows Update or using the .ISO file). Ensure the upgrade is completed successfully and then ensure that it is activated.
You can then proceed to do a clean install by using recovery media or using the Reset function in
Start > Settings > Update & Security > Recovery > Reset this PC (Get Started).