Need help with uploading code files

Site URL: https://www.abehsignatureapartments.com/

I am trying to upload custom code files (HTML, CSS, JS, Images). And I can't seem to find a way to do that. Apparently, developer mode (which I believed was a way to manage/transfer files) is only available in version 7.0 and not 7.1. I've tried using an FTP Client - Filezilla precisely - no luck there either.
Can anyone help with this please?

Hi,

Images, CSS, JS, you can follow this guide.

or add CSS to Custom CSS, JS to Code Injection, HTML into Code Block

Can you describe in detail what thing you want?