Q: How do I edit buttons in the cart?
A: Edit the file app\design\frontend\base\default\template\checkout\cart.phtml.
It is good to keep the default files untouched. In order to do so, copy the files from the base\default category to default\YourTemplate. For example, the file app\design\frontend\base\default\template\checkout\cart.phtml would go to app\design\frontend\default\YourTemplate\template\checkout\cart.phtml
Q: How do I edit the "Add to cart" buttons on the product pages? I figured it out on the category and search pages, but not on the product page.
A: Edit the file app\design\frontend\base\default\template\catalog\product\view\addto.phtml.