Go to ->.
In the tree, select the GUI item.
In the pane, find the icon property and its value.
Icon size must not exceed 20x20 pixels.
For global icons, save the new icon to the sernaInstallationPath/icons directory.
For plugin-specific icons, in the icons subdirectory within the plugin directory overwrite the icon coming with the distribution with the new one.
Restart Serna.
You can see an example of overriding an icon in Overriding an Icon.
To learn more about GUI icons, see GUI Icons.