Both ASCII and TXT files are just text files so editable using any text editor and you can rename the extension freely. However you may have been looking for a way to convert ASCII numbers to a text in which case it would be best to use websites like ASCII to text converter.
There is also the option in some better text editors to change the encoding of the text file, for example from UTF-8 to ASCII etc. In this case you have to save your text file with a specific encoding. Some text editors feature "Save with encoding" function that is exactly for this purpose.