I've got some steps on how to change the vendor name on your closed purchase order, Lia O,

We can go to the Vendor Center and edit the transaction to another vendor name. Let me show you how: 

1. Go to Vendors on the top menu.
2. Choose Vendor Center. 
3. Click the Transactions tab. 
4. Select Purchase Orders.
5. Tap the Filter Type field and choose All Purchase Order. 
6. In the purchase order's lists, double-click the transaction.
7. Select a name in the Vendor drop-down field.
8. Hit Save & Close.
9. Press Yes to confirm.

Here's an article you can refer to for more details about creating purchase orders in QuickBooks Desktop.  To run all closed/open purchase order transactions, just follow the steps below:

1. Go to Vendors on the top menu.
2. Choose Vendor Center. 
3. Click the Transactions tab. 
4. Select Purchase Orders.
5. Tap the Filter Type field and choose All Purchase Order. 
6. Tick the Run Reports drop-down and choose View as a Report.

Feel free to visit our Expenses and vendors page for more insight about managing your vendor transactions. 

I'd like to know how you get on after trying the steps as I want to ensure this is resolved for you. Just reply to this post and I'll get back to you. You have a good one.