Cath Everest update

Hello, I downloaded the Cath Everest update. I don't really know how to update it in wordpress. I use File Zilla to upload the themes, so should I delet the one that is in wordpres and upload the new one? Will that mess up the website? If I need to do a website back up, can I do it with File Zilla how? I'm still trying to make our website work and I just don't want to have a problem (I have enough to think about now including how to fix the footer!!). Thank you for ALL your help! Lorena.