Thank you for your post.
Click once on the tab-delimited file so it highlights. Pull down the File menu and select Get Info. Look for "Open with", and change the application to TextEdit or some other application. Just below the selected application, it says "Use this application to open all documents like this.". Below that, click Change All... This will then change all files with the extension to open under the new application, and all newly created files with this extension will also use this option.
Let me know if you need clarification for any of the above steps.