You log in to Plesk with Login to Panel (see Plesk: Logging in).

File Manager

• Go to Files. Your website's files are in the httpdocs folder.
• You can upload a zip file and unpack it with Extract Files.
• From each file's menu: editing (Edit in Text Editor or Edit in HTML Editor) and permissions (Change Permissions).

FTP

• Main FTP account: you change the username and password in Websites & Domains → Web Hosting Access.
• Extra FTP accounts: in Websites & Domains → FTP, click Add an FTP Account, enter a name, folder and password and click OK.
• In your FTP program (e.g. FileZilla), enter your domain or the server address as the server, and the FTP account's details.

For FTP and FileZilla in general, see What is FTP and how do I upload my files.