Launch (or converted PKHeX). Use File > Open and change the file filter to "All Files" or specific "Raw Save." Select your .rxdata file. If the editor recognizes the save, you will see your trainer card, party, and PC boxes.
If you are a developer looking to interact with these files programmatically: save editor rxdata
: Developers use tools like psdk --util=convert to turn .rxdata files into human-readable .yml files, which can be edited in any text editor and then converted back. Launch (or converted PKHeX)