How can I replace the missing es_ES.po file

I have a site that's translated into Spanish, but I wanted to change how the months show up. Currently they show up in lower case. I wanted to change them to uppercase.

I found the es_ES.po file in wp-content/languages I started to edit it in my FTP but when I saved it, it disappeared from my server. The corresponding .mo files is there, but that cant be edited. Where can I find a new es_ES.po to download? Or is it possible to convert .mo to .po?