ASC files are typically found as ASCII files, hence text based. Should you need it, you can rename the suffix of .asc files to txt, so these files are associated by text editors by default. However, it is not exactly needed as you could manually open them in any text editor regardless their actual extension, no asc to text conversion required.
Such asc to text coversion might also in broad sense describe conversion of ASCII numbers to text form, in which case you need to use a proper converter, such as ASCII to text converter.
Open ASC file Open TEXT fileAn online tool for generating ASCII number from text
One of the many text viewers and editors