Text in Moodle may be changed by editing the language strings.
To ensure that any changes are not overwritten by a new version when upgrading, you need to create your own local language pack as follows:
- Copy the contents of your language folder (e.g. lang/en) into a new folder (e.g. lang/en_local) - you will need access to server to achieve this step, it cannot be done from within Moodle.
- Make your local language pack the default for the site via Administration >> Configuration >> Variables