You need a PO editor to edit PO files which is the main format used by Free Software, it is also the format the the Translate Toolkit uses when converting Mozilla and OpenOffice.org files for editing
There are a few PO editing tools available. Your choice really depends on which platform you are running:
You may be able to use a text editor to edit PO files, since they are text:
Or you can try to convert and/or use another format: