Can someone please tell me which file I need to edit in order to replace the text heading "model" with the heading "part number"?

I went to developers tools and searched for the word 'model' in all files. I then manually edited every file and replaced "model" with "part number" but I obviously did something wrong because my Zencart pages went blank. So now I have re-uploaded all Zencart files again.
I don't want to make the same mistake again.

Do I only edit a particular file or all of them?
Also, should I put "part_number" as opposed to "part number"?