Programs enabling conversion of the file:
If your objective is to convert the file from M4A to the file M4R, you can use any of the applications listed. M4A to M4R conversion process in every program intended for this purpose (so-called converters) is very intuitive and everyone is able to manage converting a file.
However, bear in mind that the list may also include programs to support and work with the indicated extensions M4A and M4R - and not only converters. In such a case, you need to open the source file M4A and from the options available in the program, choose ”Save as... M4R" or "Export to M4R".