Dallas here from the Shopify Social Care team. I'd love to help answer your question.
Unfortunately, the text on the checkout button isn't able to be changed. This would require a bit of a coding adjustment and the code sheet for the checkout is actually a locked page. Our theme support can't edit it and external Shopify Experts won't be able to edit it.
The alternative that I have for you is to change the color of the button. It might not be what you're wanting, but if you want more/less contrast between the two colors you can go to your Store Admin and click on Online Store > Themes > Customize.
From there you'll want to click on Theme Settings and then go down to Checkout. Partway down the page, you'll see an area that says Buttons. That won't change the text, but if you ever want to adjust the color of the button you can find that there.
I know that's not exactly what you're looking for, but I hope that helps to shed some light on the situation and clarify a bit for you.
If you have any other questions please don't hesitate to ask me.