Both ASCII and TXT files are just text files so editable using any text editor and you can rename the extension freely, no ascii to text conversion required. However, it might be very well possible that you were searching 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.
Open ASCII file Open TEXT fileA text format conversion utility
An online tool for generating ASCII number from text
One of the many text viewers and editors