We recommend using a free program called "Poedit" to make changes to your translation files, and this article will go through the steps of using it. You can find download it on most operating systems at http://poedit.net/
The first thing you'll need to do is setup your identity on Poedit by going to File - Preferences. You will need to fill in your name and email address in order for it to save properly.
After your identity is setup, you can open /language/core-es.po - this is an example, you will open the .po of whichever file you want to edit. Once you've opened it, you will see that the existing translations are found at the bottom of the file and the terms that haven't been translated are found at the top. You can click on a phrase or word that hasn't been translated, and simply provide the translation in the box below the word.
Before we save our changes, we need to establish our team because this is how Poedit works, it adds this information to the language file to show a history of who has last changed the file.
Navigate to Catalog - Properties and add whatever team name you wish.
Lastly you will need to save the file. When the file is saved, it will also save a .mo file, such as core-es.mo - this is the file you need to upload to your website in the /language/ folder and replace the existing file. You've now modified the language translation!