In this documentation, you'll learn how to change the Coupon success message on a Checkout page.
Here's what we're going to edit:

Follow along:
Step 1: Go to the 'Fields' tab in Checkout
On the Fields tab, click on the 'Coupon' field to start editing it.

When you do so, a popup shows with a bunch of options to customize the coupon field.
Step 2: Edit the Coupon success message text

Once done, press the 'Update' button.
Step 3: Hit the 'Save changes' button

This will save your changes.
Here's the final output:

This is how you can change the Coupon success message on a Checkout page.