Change labels checkout form

When in the checkout you get the little radio menu about what you want to do with the coupon.

– Send it to me
– Gift to someone else

When you click the second option an email field and a message field appear with placeholder. I would like to know how I would be able to change the text in these radio labels and placeholders. I can only change it now in files that will be overridden on the next update.

Kind regards