You can simply exchange the file ending. CSV is just a special type of a text file. The content is tab separated.
Alternatively you can go to the menu at File / Export / CSV. In the file dialog you click on the subset button and select the are you want. Then save the file to disk. This will be a bit different format. But it is also CSV and it has this file ending.
For details of this format, please have a look at the help:
