Open a web browser and type the IP address of your Yeastar S-Series PBX system.
Enter your username and password to enter the management portal.

When inside the Dashboard, look at the left navigation sidebar. Click on the gear icon labeled "Settings".

Click the icon labeled "Security" under the System group of Settings.

The Security submenu has various settings grouped in tabs. Select the tab named "Service" (top-right).

Select Enable FTP (keep the default port setting of 21).
Then select Enable SSH.

After selecting "Enable SSH" a new window appears showing the current ssh/ftp password. You can press the "Regenerate" button to create a new password. Write down the new password and don't share it with anyone. Press "OK" to return to the "Service" options. Now you can untick "Enable SSH" if you like. The generated password will work for FTP access with or without SSH. Finally press the "Save" button.

Select the "Storage" submenu under "System" in the Settings window.

Select the "File Share" tab of "Storage". Make sure that "Enable FTP File Sharing" is ticked. Then push on the "Save" button.