you should be editing the template for this.
if you are editing the template, but the change is not being reflected, there are a few possibilities:
-You are editing a different template than is used by the theme that is applied to the page
-The logo is an editable region in the template, so will need to be edited on the pages themselves
-The page is using the template, not the theme, in this case, you need to apply template changes to the page using the Modify -> Templates -> Update pages menu item.