Language Overrides in Joomla
From Joomla! Documentation
To change text in core Joomla or in an extension (other than installing a new language) it is advised to perform a language override in Joomla. To do this you need to go into the admin backend then in the menu under "Extensions" click on "Language Manager".
Then on the submenu on the page you come to click on "Overrides" as shown in the image below.
Click on "New" in the toolbar. Here you can change any string in Joomla. You can use the box on the right to search for a string that appears in the front end. Or if you know the constant you wish to override you can immediately enter that in the box on the left-hand-side.
Once you have your string you enter it into the "Language Constant" box on the left of the screen. Then type in the textarea beneath the text you want to display. Once you've done that click "Save and Close" and then repeat the previous step for each of the strings you need to override.
This may seem like a hard way of doing it rather than simply editing the language string files - but the advantage is that when you perform an update an extension/Joomla these strings won't get overwritten and generally, it's easier to do it this way once than it is to do it every time Joomla/the extension gets updated and having to edit the .ini file again.