It's similar to what you had to do when you uploaded the file to authenticate your Google webmaster's account but in reverse.

You start up your FTP client and navigate to the file in question in the RHS pane of the S/W. You then download said file to your PC and open it in notepad or similar, make your edits and save the file. You then upload the file back to where it came from using the method described above but in reverse. It sounds more complex than it actually is, once you've done it once you're find it easy. Make sure you have a backup of your site before hand in case you make an error.

You could do with some website editing S/W but I'm our of touch with the free/cheap ones. I use Dreamweaver which is overkill for what you need, not to mention expensive. I'll see if I can find something suitable or perhaps someone on here can recommend one that will suite your needs.

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Lawrence