The Redeem Prompt text is the text in the popup that appears when you click the Redeem Button on a Coupon Page (there's an option to disable the prompt). The "Are you sure?" is an example in the below image:
To edit that, you need to edit your Coupon Page in the Pages section. Once you're in the page setup wizard, go to Step 2 (Properties) and open up the Redeem Button item/element by clicking on it.
Scroll down just a bit and you should find the Redeem Prompt option. That's where you can edit the redeem prompt text and you also have the option to disable the prompt.