Windows 10 Media Creation Tool is a portable program which provides various ways to update a PC to Windows 10 build 1809 or, if you're a Windows 10 user, get hold of the very latest version (October 2018 Update).
You can use the package to install Windows 10 on the current PC, if it's running Windows 7 or 8.1, or reinstall Windows 10 on a system where you've already activated Windows 10.
There are also options to create a bootable environment (a USB key or ISO file) for installing Windows 10 on another PC.
Please note, there are several points to remember here.
Build an installer on a USB key and any existing data it contains will be overwritten.
If you've not installed Windows 10 before, you'll need a product key. (If you're reinstalling we'd make a note of your current key anyway, just in case.)
Microsoft also warns that "if you have Office 2010 or earlier and choose to perform a clean install of Windows 10, you will need to locate your Office product key."
There are more instructions on the Media Creation Tool page, if you need them.
A handy way to build your own Windows 10 installer.