In some cases we need to update an item that already exists for example:
The user performs a task, the task is tracked by the workflow which runs from the task list (list_1), as changes to the task occur another list item (list_2) needs to be updated.
List 1 and List_2 shares a common value (ProcessID) which is generated earlier in the workflow, but the list’s has different list ID’s.
The List_2 item is not created in this workflow, and for that reason you cannot use the Content Type approach.
By using the “processID” that is common on both fields you can update the item in List_2 as follows:
1.Use the SharePoint Search Results Event Wizard a.Choose the list you would like to search 2.Add a search filter (I used the following criteria in my process)
  a.Content Type : All
  b.Field: ProcessID
  c.Operator: Equal
  d.Value: ProcessID (use valid ProcessID, either hard coded or from XML Data Field)
3.Store the results in a content field. (I used the following criteria in my process)
  a.Type: List Item
  b.Content Field: Results (created new one)
4.Finish Search Event This will save all the item information to your created content list called tResults] where the ProcessID is equalled to the value specified in the Search Event.
You can now view the data structure of Results] in the object browser.
Within the structure you will be a tag wID]< item> containing the item ID of the list item.
5.Add List item Event to you process for updating the specific list _2 item a.Select Update List Item 6. Add following criteria for Update List Item wizard
  a. URL: rYour SharePoint URL]
  b. List Name: List (List_2) where item resides
  c. Content Type: Item d. List item ID:
      i. Open Object Browser > XML Fields > Process > Results > Items > Item > Item 7. Assign the value that needs to be changed.
8. Finished
This will update the List_2 item pID]< item> which was return by the search using the ProcessID.