You can export products with this fields (Internal Reference, Name, Expense Account, Income Account) to spreadsheet and select "I want to update data (import-compatible export)" in export window, this will export products with their external ID (This ID will be used to update the products without duplicating them so don't delete or change it), Open the spreadsheet and fill the  Expense Account, Income Account and save.  Then you can import the spreadsheet to products again.

More details in Odoo Documentation

Can I import several times the same record?

If you import a file that contains one of the column “External ID” or “Database ID”, records that have already been imported will be modified instead of being created. This is very useful as it allows you to import several times the same CSV file while having made some changes in between two imports. Odoo will take care of creating or modifying each record depending if it’s new or not.

This feature allows you to use the Import/Export tool of Odoo to modify a batch of records in your favorite spreadsheet application.