Add a link on to /cart on /cart/checkout, using uc_cart_checkout_form. necessary or not? I am trying to add a link or button from the checkout page 
http://dev.hearst.gotpantheon.com/cart/checkout

To the update cart page
http://dev.hearst.gotpantheon.com/cart/

Right now there are only two buttons showing up, "review cart" and "cancel"

strangely enough if I press "enter" on my keypad I get redirected to the desired update cart page.
http://dev.hearst.gotpantheon.com/cart/

I am thinking of using 

function MyThemeName_uc_cart_checkout_form($form) {

}


To add a simple link to 'update cart' into the html or as a form element, I am new to Drupal. How would I do this using the form API? Or is there simply a setting I can set to make the "update cart button" appear http://dev.hearst.gotpantheon.com/cart/
The cancel button will take the user back to the cart page, which is why you are redirected there when you hit enter on the form.

You could indeed add the link by theming the checkout form.

The alternative would be by modifying the form itself, and adding a new markup type element:

$form['cart_link'] = array('#value' =&gt; l('Cart', 'cart'));


You can modify the form using hook_form_alter

But using the theme hook is fine too!