The most efficient way to access official ISO files is through the official Media Creation Tools which Microsoft hosts. These files can also provide the ability to create USB drives for the same purpose and also can directly allow users to install new versions onto their properly working systems without any intermediate steps (cutting an ISO to disc or creating a finished USB drive.) Another perk is that ISO files from a Media Creation Tool are significantly smaller than those which can be gotten upon release from Microsoft. Those ISO’s are larger than a standard single-layer DVD can hold and must be cut to a dual-layer DVD. The ones that result from a Media Creation Tool are less bloated but still perform the task properly.
Please also note that Media Creation Tools are available for all supported versions of Windows 10 at any particular time and can be used to upgrade to the next version or several versions ahead if you’ve held off on upgrading. They can also downgrade a system to an earlier version as well but you should be aware you will probably lose some or all of your (non-cloud) data along with your installed programs and their settings when downgrading. Be prepared and backup, backup, backup obsessively before doing a downgrade.
The best way to find available Media Creation Tools is to search Bing for “Media Creation Tool” along with the official version identifier which Microsoft has announced (for best results). Choose only microsoft.com links when selecting Media Creation Tools or, for that matter, technical data or help pages. There is plenty of good, helpful information on other sites but everything there needs to be looked at closely and evaluated thoroughly for the possibility of harmful downloads.
If you run an older version, remember you won’t get any Windows Updates past a certain point.
Download the Media Creation Tool for the version you want from the links below — it will let you create an ISO image (which you can burn to a DVD, or use to install inside a virtual-machine) or a bootable USB drive (the drive must be 8GB or larger, and everything will get deleted off it in the process, so check there’s nothing you need to keep first!).
1507 (use the first link if you want 64-bit Windows 10, the second for 32-bit Windows 10 — in all later Media Creation tools, you can choose inside the tool itself which “bit” you want):
download.microsoft.com/download/1/C/8/1C8BAF5C-9B7E-44FB-A90A-F58590B5DF7B/v2.0/MediaCreationToolx64.exe
download.microsoft.com/download/1/C/8/1C8BAF5C-9B7E-44FB-A90A-F58590B5DF7B/v2.0/MediaCreationTool.exe