Hi Alexis:

I'm interpreting your question as you want to update the traceability of a large number of products and set it to "By Unique Serial".

The fastest way to do it is to go to Inventory > Master Data > Products, switch to the List View, select the Products you want to update and Export the data to an updatable csv file of just the "Tracking" field like so.

This will create a csv file with 2 columns in it - id and tracking. Update the values in the tracking column to By Unique Serial Number and import the file back into Odoo.