Total clear and reset an HP stream win10

I have inherited a computer that I do not know the passwords to. I want to use “reset this pc” and “fully clean the drive’ When I get to the stage to start I get this message=”additional free space is needed on the drive where windows is installed-free up some space and try again” Obviously I cannot free up space on any drive if I cannot access the computer. Windows 10 O/S. Any assistance would be greatly appreciated. Thank you.