Hi Steve, I've just quickly had a browse at the template you are thinking of buying. To be honest, and from my experience as a web designer the template doesn't look great for the price of ~£25. I would recommend shopping around e.g. Template Monster click here or just search Google for free templates.
However, if you do use this template then to change the menus should be straight forward. I assume they will be provided to you as PNG files so just load them up in Fireworks. Select the button/menu, change the text on it and change the hyperlink (this should be displayed at bottom of screen).
The best thing is to have a play and if you have any specific problems posta message here or email me.