Let's say I have two lists, List A and List B.
List A has a field named Quantity A.
List B has a field named Quantity B.
I want to create a workflow on List A that, when I add a new Item in List A, updates the value of Quantity B from List B like this: Quantity B = Quantity B - Quantity A.
Is that possible using Sharepoint Designer?
Yes, that's possible.
Use the workflow action to update a list item. Follow the dialog. You will need to identify the list and the list item that you want to update, but it's pretty straightforward.
Edit: Use the "Update List Item" action. Click this list. Select the list you want to update. Click Add and add the fields you want to update.
In the "Find the List Item" section select the field you want to use for identifying the correct item, for example the ID field. In the Value box enter what the value of the field needs to be, for example 3 for the item with ID 3.
Click OK.
