User Defined Targets?

Hello!

I am trying to learn Vuforia by following a book. The book mentions a UserDefinedTargetBuilder.prefab which I guess is discontinued because it's nowhere to be found. I went ahead and tried to implement it's behaviour. So far I've managed to get a screenshot and create a new imagetarget. However, this means that a new imagetarget is created everytime the user clicks a capture. I can't seem to be able to change the image source of an existing imagetarget. How can I accomplish the same behaviour as the old UserDefinedTarget.prefab?