If you're:

1. Editing it properly (in a text editor).

2. Saving it to the proper override folder ( if you've selected the 'classic' folder in Admin - Template Selection, then you want it in includes/languages/english/classic

3. Making sure that the permissions are correct, such that you're actually sending and overwriting the file with your FTP clent

then it has to work, which means one of the above isn't happening.

If you post a link to your site, we can take a look.

Edit: afo is right, it might be wise to create your own template first.