You've downloaded a Tableau file and not the datasource. It seems you've come across an education portal or some community support sites since there'll be no other case where any publisher will allow you to download files in twbx format.
1) Look on the portal again, the person might have shared the datasource (in the csv, xlsx etc) format along with the dashboard (.twbx files)
2) Go to the sheet whose data you want to export. Click on:
Data --> [Name of Datasource from the dropdown] --> Add to Saved Data Sources...
You can save the file as "filename.xlsx" and you'll be able to access it through MS - Excel
If you also want to learn Tableau and make your career in it then I would suggest you must take up the following Tableau Certification course online.