How to change password of other pc using ip

Hello,
how can i change password of other pc using ip address in cmd??the pc i want to change the password is not in my network.i use router for my network and my router is connected to switch hud that connected to internet and the pc i want to change the password is connected to switch hub.pls help this pc is for my sister and my cousing always use it without asking my sister..i want to chnge pssword without contact to that pc to know that they are hacked by others and stop from using it.ty