I am not understanding the problem. If you have established a file association for a txt file, or any other file format for that matter, double clicking on the file will start the program required. If you need to change a file association, open Windows Explorer Tools>Folder Options>File Types. Delete the file type you need to change. The next time you attempt to open that type of file, windows will allow you to choose the program you want to use.

I assume that you know everything I have said, so you must be dealing with something more obscure.