You can consider downloading tools like Hiren
Once you had downloaded, you can have the option to burn it into a cd or to usb thumbdrive (preferred as it is fast).
You will need another working computer to do so.
There is a tool in hiren you can use to format it.
Hard Disk Tools – Security
HDD Low Level Format Tool v4.40
For a less secure method, you can download windows 10 installer from microsoft, once download is completed, you can either burn it to a disk or usb, which usb is preferred as it is faster. boot it up and delete all partition. After which, you do a clean windows installation over.
For a secure erase, i know there is a tool which can help, but it is not cost effective for you, like wibetech hdd eraser, without a need of pc.
else if it is a concern, and there is no much option left, i would rather you remove the hdd