The root directory for your website is htdocs. This can vary if you have multiple domains as they will be under your-domain.com/htdocs/. To check what the root folder for your website is, you can go to the client area and go to the Domains page:
It depends on what you mean by “root directory”. It’s sometimes used to refer to the home directory or your account, or the so-called Document Root of your website.
If you need to upload files, it’s probably for the purpose of uploading something to a website, which means it should be uploaded to your website directory.
To see which directory that actually is, please check the Domains list in the client area. Next to every domain name in that list is a “Directory”. That’s the folder on your account to which that domain name is linked. So if you want to upload files to that website, you should upload them to the directory that domain is linked to.
Sometimes that’s the htdocs folder, sometimes it’s a different folder. So if you’re not sure, please check the Domains list so you know exactly which directory you should use.