You can change the default postings for products used on travel and expense invoices from the Management > Company info > Posting defaults > Default postings for travel and expense invoices view.
First select the product to be edited and click on the Change account button, which will open the Account list page, on which you can pick the desired account. You can select more than one product at once by using the Ctrl and Shift keys.
The Reset to Procountor defaults button restores the Procountor default accounts to all products.
If the option Travel and expense invoice posting / Post invoices by row by default? has not been selected in the Usage settings in Company info, you will not be able to edit the default postings of travel and expense invoices.