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There are 2 options of adding favicon
1. Easy way if you have .ico image
If you have favicon.ico image you can replace this file using FTP or Plesk access in /httpdocs/folder.
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2. Advanced for any image format
Use any favicon generator service to make images for different browser and device types. For example, https://realfavicongenerator.net/
You will receive a number of images and instruction what to insert into HTML file. Use FTP access to template to open pageheader.jsp and put HTML tags there
upload images using FTP accesss