To do this with the use of the "Update All" functionality is also possible. Write down the ID or the searchcode of the "Master CI".
Go to advanced find, go to tab advanced press "Add criterion" and then *TYPE*
Related CIs;CI From
once you type that the "Operator" and "Value" fields will be enabled. As operator choose "equals" and at the value type the ID or the searchcode of the "Master CI". Press ok to add the criterion and press "Find now". All CIs that are related from "Master CI" will be listed. Then use "Update All" to set the category.