Yes, it's to stop someone freeloading on her wifi network ID.
I really don't think there will be a problem of this person using your wifi unless he sits outside in his car or lives next door.
I have a sky router and I must admit i don't know how to reset the password
If your real worry is that this person might be able to see your PC because you gave him the log in details (as per your earlier post) then I don't see how he can unless he can get hold of your PC. Certainly changing the PC password for peace of mind is easy
If it is the wifi password you need to change then i am not sure it's necessary but if you want to do it then please tell us the details of your internet provider.
To change your router's password you need to access its settings via your web browser. If we're to tell you what to enter in the address bar of your browser (and the default password you'll then need), we'll need to know the make and model of your router.
If you do a web search for your router then you can probably find the IP address you can type into your browser address box to reach your router's management page. Log in there and you can change all sorts of things. But be careful what you do change and make a note of what it was before you changed it, just in case you end up wanting to put it back as it was.