Hi Jane,
Usually this type of customization is pretty easy to do as it is just a matter of finding the right language string to change. In this case it will also depend on the enrollment method as each one has it's own language string, so you may need to change more than one.
Go to Administration > Site administration > Language > Language customization, chose the Language pack and then Open language pack for editing, and finally Continue.
In the drop down list for Show strings of these components, scroll down to the enrol section, click on the first (enrol_category.php), hold down the the shift key and select the last one (enrol_self.php) and enter the word, expire, in the Only strings containing box. Click on Show strings.
Look through the resulting list for the "Message" you want to change and type the new version in the box next to it. At the bottom of the page click one of the two buttons to apply the change.