How do I update a set of records using a Modify Data process element?

I have a configuration item with a number of children.  All the configuration items (parent and children) are associated with an Account in the CI User table.  I am writing a business process to transfer all configuration items from one account to another. 

In SQL, I would do something like this:

Update CI User

Set Account =

Where Configuration Item in (Select ID from Configuration Item where Parent CI = )

In the Process Designer, I tried using a Read data process element to read the IDs of all the children Configuration Items, but when I try to reference those IDs in a Modify Data process element, it will not let me select any fields for that Configuration Item. 

Maybe this is a basic question, but how do I update a set of records using a Modify Data process element?