This is Max from the Shopify Team.
Manipulating the code of a checkout page within Shopify is not something that you have much access to unless you are a Shopify Plus account. The reason for this, is so we can make sure checkouts meet security compliances as that's where important information is entered and buyers are directed to their payment gateways.
Have you thought about adding a continue shopping button to your cart page instead? This is the last page most shoppers would see before continuing to the checkout! Some themes have this option built-in (like these), so you could add it by adjusting the options in your theme editor (Online Store > Themes > Customize). Otherwise, you might find that this tutorial - Add a 'Continue shopping' link to your cart page is useful.
In terms of apps, most options offer pop-ups with a continue shopping and checkout option, like these; About One‑Click Checkout and Globo Straight to Checkout.
I hope that this helps explain some options a bit more! If you have any follow up thoughts or want to share the approach you took and get some feedback, please let us know.
Max | Social Care @ Shopify
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Even without access to /checkout/ there is plenty of scope to test different functionality, copy and layouts from the product page CTA button/s, popups, fly-in carts, /cart/ pages etc.
Also, an app for this sounds like overkill. A button like that should literally be a couple lines of liquid code if you set it up properly. Also - all those touchpoints in the checkout flow are a great target for A/B testing.