I have been given an older Apple computer (the kind where everything is in the screen case), but I cannot use it, because it is linked to the Apple Id of the previous owner, who I have no means of reaching anymore.

I have already tried to reinstall OS X (El Capitan) from USB stick (which I prepared on a Windows PC following instructions given in another thread here), but when that was done, I ended up at the login screen requiring me to enter the previous owner's credentials.

How do I get rid of that, so that I can link that computer to another Apple Id?