Hey Lorraine,
Great site - your products are fun!
There is not currently a way within the app to bulk edit the MPN however you can go into your Products admin and bulk edit the Barcodes. To do this select all the products and click on 'Edit Products' so that the bulk editor loads.
You can then select 'Add more fields' and choose 'Barcode' to open up the Barcode column.
To do the same for taxes, choose 'Taxes' from the 'Add more fields' dropdown to open up a Taxes column for quicker editing.
Kindly,
Danielle
Shopify Guru