Hi Dean,
You should be able to edit almost any string of text displayed in Moodle, by editing the string in the language pack using Moodle's built-in language customization tool.
For more information on how to modify strings in Moodle, see:
https://docs.moodle.org/en/Language_customisation
Note: This will likely change all instances of this string throughout your Moodle site, not just in the user's profile and login page. This can sometimes produce unexpected results.
Hope you find this helpful.
Best regards
Michael Milette