Can you be more specific about what you mean with regard to "website value"? If you mean change content of a web page, you need to find out where the page is being hosted. Connect to it via FTP, grab the file, make modifications, and re-upload to the hosting provider.
You can start by searching the domain name with WhoIs. This will at least tell you who has registered the domain with their contact info. You can then ask them about where the site is hosted.
I was told to find the registrar(?) and to change it to a different number. I have no clue what the old numbers are, who made the site, how they made it, when. This is the task I have been sub-given. (I am helping a guy, who is helping them)