Website builder encrypted all files and use your current FTP login and password as login info which gets baked into your site along with your site URL.
You can't change the file using an external editor or your site will not load because it was corrupted by the external editor. You can only change your website builder inside your website builder via admin URL.
You need to login to your website builder with the original FTP password when you create the site or your own custom login if you change it in website builder Config, Access tab.
. You need to make sure you change your website builder login and password in website builder to something you remember via Config and Access tab otherwise when your FTP password change you can't login to website builder.
. If you don't remember the original password then your only option is to reset, rebuild or upgrade to paid host to avoid this limitation and have a better website builder.
. If you decide to change your template it is like you reload your site which you will loose all content and customization that you have done to your site.