How to override standard page message?

Hi

I’m looking to override some of the standard page content or messages.

I installed the Simple Membership Custom Messages plugin but that doesn’t go far enough.

I want to be able to change the text in some of the content in the file class.swpm-front-registration.php but obviously don’t want to change that actual file because the changes would get overwritten on the next update.

So this is text like “Free membership is disabled on this site. Please make a payment from the ‘ . $joinuspage_link . ‘ page to pay for a premium membership.”

Is there any way I can add a template file that wouldn’t get overwritten (to my theme or to the plugin) or anyway I can do this in my functions.php file?

Many thanks

Allan