Child Theme Newbie

I’m a newbie with child themes and I have installed one for the Responsive Auctionpress theme. I watched some of the videos on child themes but still a tad confused. I’m trying to edit some wording which cannot be found in the Admin under Language the wording can only be changed in framework/class/class_auction_defaults.php

So if I wanted to make changes to that file without affecting the future updates how would I go about this the correct way?

Thanks,
James