link new case to original case HOW???

hello

I am using a workflow to create a new case that is an exactly replica of the original case

so I have a step that creates the new record

I must then update original record to link to the new case

I have a lookup that does this but the original links to itself and not the new case

how do I do this via a update step

again I use a workflow to create a duplicate of the case I then I create a step to update case but I need to link the new case to the original

how do I do this