To mass change/update your products, you export to excel (or csv if your prefer). You select the option: Update data (import-compatible export), and Odoo add the external ID.  If you look at the meta data for a product record, you can see it as XML-ID, I have learnt.

There is a big BUT! Assume your products has been manually created in Odoo, and not created by import: If you do export from menu option: Inventory->Masterdata->Products the "External ID" looks like this when you export: .1720034.  When I import, this value is not recognized, and the result is that new records are created.  Bad.

The trick I discovered this morning: Go to Inventory->Masterdata->Product Variants.  Then the "External ID" or XML ID looks something like this: "__export__.product_product_88_93ee1d10".  You can see the difference if you look at Metadata.

According to the Help file for import of products: If the products has been manually created, Odoo will create a new unique External Id when you export.  So my experience is that for products this only happens if you export from the Product variants list.

It took me about 4 hours of frustration to discover the difference, so hope it might help somebody...