Option 1... Reset the computer...
Windows 10 actually has a built-in method for wiping your PC, and restoring it to an ‘as new’ state. You can choose to preserve just your personal files or to have it erase everything.
To do this go to "Start", then "Settings". Next choose "Update & security" and then "Recovery" Here click "Get started" and select the option you wish to use. From here just follow the on-screen instructions to restore Windows 10 back to how it was upon leaving the factory.
Option 2... Securely format the computer...
If you’re selling, or giving way, your computer to someone else, then it is better to securely erase the hard drive, before reinstalling Windows 10. This will prevent any old data from being recovered. This, however, is a bit more work.
Also, if you don’t have a Windows 10 install disc or USB of some sort, you will need one.
Now there is are programs you can download to erase the drive completely. One common one is DBAN (Darik Boot And Nuke). You would need to download it and then burn it to a cd or USB.
Then when you are ready, boot your computer up using the DBAN disc or USB, and hit "Enter".
When the main DBAN screen appears, use the "J" and "K" keys to highlight the hard drive partition you want to erase (if there is more than one that is) and select it by pressing the "Space bar".
Then, when you are ready to proceed and erase the hard drive, press the "F10" key to begin the secure erase. This will take some time and the process cannot be interrupted.
When the secure erase has finished, and you see a ‘Blancco’ ad, eject the DBAN disc/disconnect the USB and reset your computer.
Now reinstall Windows and you should be good.