I'll assume that you've not got a copy of your website stored locally on your computer. (If you have, you can still use the following instructions but simply omit the bit about downloading a file):
To pass information between your computer and the server where your website is hosted, you'll need an FTP client. The free version of CoffeeCup will meet your needs:
http://www.coffeecup.com/free-ftp/
Open CoffeeCup and use the login details you've got to gain access to the files on your website. (You'll see those files in the top right-hand panel. Files on your computer are shown in the top left-hand panel; set it to point to 'My Documents' or somewhere similar).
Select the relevant file (where your contact details appear) in the right-hand panel and then click on 'Download'. That will copy that file to your computer. Then open it in any HTML-editing program. (There are lots of specialist programs available but Microsoft Word, or any similar word-processing software, is perfectly adequate here). Edit that document, to show your revised contact details, and save it.
Now return to your FTP program, select the revised file and click on 'Upload'. You'll be asked if you want to over-write the old file, so (obviously) you select 'Yes'.
Job done.