How To Change Icon For Gift Certificate Submit Button?

We are having issues with customers not applying a coupon/GC code once they have entered the code on the view cart or checkout screen. We are on version 4 and currently it uses a glyph set, but we want to change it to the word "Apply". We have created the png, but cannot figure out how to swap it out. Anyone have an idea on what template and code to change?

Another this is this button is also linked to the newsletter signup submit button, but we want to keep this button as is.

Screenshot attached for reference.

change-apply button.png

Could you just write a style to overwrite for that specific container? (.coupons-container in my version)

.coupons-container i.glyph-class-name:before {content: url (path/to/your/new/button) !important;}