When you open the portfolio on top of the portfolio table, three buttons will allow you to export/import a portfolio.

Exports are possible in both Excel and CSV format, and will provide an export of your current selection in the portfolio.
Via the Import button you can upload a new set of third parties to your current portfolio by uploading a csv document containing DUNS number (mandatory), Client ID (optional), Tag (optional).
You can also choose to modify existing records in your portfolio if you want to change or add Client ID or Tag to a set of third parties. You must upload a csv document containing DUNS number (mandatory), Client ID (optional), Tag (optional).