Windows 10 Media Creation Tool is a portable program which provides various ways to update a PC to Windows 10 build 1903 or, if you're an existing Windows 10 user, get hold of the very latest version (May 2019 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.