Download the Windows 10 media creation tool, if you search Google it should be one of the first hits. Run that, it will ask if you want to update this PC now or to create media for another computer. Insert a USB flash drive that does not have any good data on it (the data will be lost), and tell the program to create media for another computer, and to create a USB flash drive.
After it finishes, reboot your computer and boot from the flash drive, then do a fresh install of Windows 10. If your computer has proper license, it should pick up your Windows if it's newer, or you may have to enter a serial number if you have one attached to the bottom of the computer (some older or refurbished systems may have this).
Let it install Windows and then you will need to install all drivers, updates, antivirus, etc. Make sure before you reinstall, that you back up any files you want to keep as the proper way would be to erase the drive when you do the fresh install.