That’s easy. On the main window, click the cogwheel next to your deck and hit export, then select Notes in Plain Text (*.txt):
Each note will have its own line with tab-delimited felds, so you can just copy that into Excel, make your edit, then export the spreadsheet as tab-delimited .txt file (Microsoft Support: Import or export text (.txt or .csv) files) and reimport it with Anki (“Import File”-Button on the bottom of the main window).