If the value still shows the previous value after you update it, caching is likely the issue. Beyond that, make sure you're editing the proper language file. So, make sure you've cleared your cache after you make any changes.
There are also many different strings with similar values, so ensure you're getting the right one.
Finally, you can also use the Joomla language manager to implement overrides for strings as well. Those overrides will take effect after you update the module as well.
I hope that helps,
Alex