I have parent and child list, in both lists, the column/field is lookup field only. whenever an item is updated in parent list, i have to update the lookup field(item already exists) value in child list with the same value(parent).
I am only talking about existing tiems in both the lists.
This is achievable.
You would have a workflow that started on item modification in the parent list.
When an item is updated the workflow finds its pair in the child workflow and updates the lookup.