You need to be more specific with what action you’ve taken for us to be able to give you the best advice. What manipulations have you tried? The easiest options are to use a plugin – like loco translate and edit using the Po/Mo files.
You’ll see the translations you can set in the loco translate plugin admin like this – http://screencast.com/t/nnnZ0n03TCc
And can check the files are saved in the lanuage folder – http://screencast.com/t/tcc7IJDUBXKF
Then that they appear live on the site, once you’ve set the site language in wp-admin > settings at the bottom of that page there’s a lanuage dropdown option – http://screencast.com/t/jJLjJjNX
Or you can open the rf_featured_content_widget.php file on your cpanel and edit the text for the read more button manually on line 60, for example here I have changed it to Lees Meer – http://screencast.com/t/jsuaZhccJRRD