OK, you need one of the above mentioned software packages, WS-FTP will prob be easier to use than IE. You need to know from him where his web site is hosted so you can FTP to the server and upload the updated files, and his login info. It shouldn't be hard once he gives you the info, he should be able to tell you the ftp path he uses, like ftp.somedomain.com/users/mydirectory/
I don't know what you are trying to update, this is just basic stuff to add/replace files. i.e. you might have to download the html file edit it and upload it. Good Luck!