I cannot find a tutorial re assigning categories to payees. Can someone please direct me?

I have 100 data entries for Payee: Trader Joe's all assigned to the Category: Groceries. I want to reassign all of these entries to the Subcategory: Groceries:TJ's. I see how to have this subcategory assigned going forward into the future, but I cannot figure out how to recategorize past data in mass. (of course I can edit one by one, but surely...) Thanks -Dennis