Most of the time, any 'button' you have on your site is an image that's been placed in a module.  Making a hyperlink on an image (so that the user is directed to a certain URL when they click on the image) is just like making a text hyperlink.  What you need to do is open the module in which the button you want to modify is located, and change the URL of the hyperlinked image inside the module.

Go to:

1. Modules > Site Modules
2. Open the module you want
3. In the editor area, click on the button/image that you want to change the link for, so that it is selected
4. Click on the Insert/Edit link icon in your editor toolbar (it looks like a link in a chain)
5. You should get a small popup window that shows you what URL or page the image is currently linked to. Cut and paste the new URL over the existing one in the field at the top of the popup window. Make sure that you include the full URL, starting with "http://...."
6. Click Update or Insert. This will close the popup window.
7. Click Apply or Save on the upper right corner

Do this for each image/button for which you want to update the hyperlink.