How to customize the favicon of a Plone 5.2 site?

Plone Support Customizing Plone
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2

To have something you can use for all your sites using the same theme, do this:

In you theme, modify rules.xml (can also be done in index.html) to something like this:

  <after theme-children="/html/head" >
    <link rel="shortcut icon" type="image/x-icon"
      href="{$portal_url}/favicon.png" />

Then you can just add an image 'favicon.png' to the root of your sites.

If you just want to do it for one site, maybe this (you just need to duplicate a theme and replace an image… not all the other stuff described ):

https://training.plone.org/5/theming/ttw-advanced.html

So: No matter how you do it, you basically just want to replace an image with another