First, make sure your computer meets the minimum spec requirements. You need a processor capable of reaching 1 GHz and at least 1 GB of RAM. Storage-wise, you need 16 GB for 32-bit Windows and 20 GB for 64-bit. You also need a GPU supporting Direct X 9 or later.
If you're using Windows 7, then pressing the Windows button at the same time as the Pause Break key in the top right section of your keyboard will give you specs.
In order to install Windows 10, you need a valid license for the OS. There are a few editions to choose from, ranging from basic packages to professional bundles that include more features.
To purchase a license, visit the official Microsoft website and choose an edition for your computer.
If the machine you want to upgrade has anything important stored on it, you should back up the critical data to an external drive. Upgrading the existing computer to Windows 10 will wipe anything it currently has stored.
If you've determined the machine meets the minimum spec requirements and it's okay to wipe, connect the Windows 10 installation media you created by either inserting the installation disk or plugging in the flash drive.
When you reboot, the Windows 10 installation will begin. Follow the prompts to configure the OS with your own custom settings and complete the setup.
If the installation did not begin, make sure the boot order in the BIOS of the target machine is set to prioritize the disk drive or USB port.