I'm trying to exist an existing website and I'm having trouble

I'm not sure if this is the right place to ask this kind of question, but I'm in need of some serious help. I have a relative that wants me to just edit the text and images of his existing website, which is hosted at aabaco small businesses (aka old yahoo hosting) and when I try to edit the code through manage files, The code between for example appears in random symbols and characters instead of being displayed in japanese. The website has no problems and I've tried editing after downloading the files through ftp but it still shows random characters where japanese characters should be displayed. Please help even if its a stupid question but this is driving me completely nuts.