Hi Darren,
Is this download button part of a form, or is it an inserted image on the landing page?
If it's part of a form, click the form and then click the wheel that appears and hit "edit", in there, you can designate a different follow up page upon form completion.
If it's an inserted image, select the image, and in the upper right corner, click the downward facing arrow in the property sheet tab and locate the linkUrl. Replace the URL in that space with the page you want to image to link to.
Hope this helps!