Work your way backwards. If you have a card with the last database
update on it, start by analyzing how the data is stored there. That
should be the first step in any project like this, the format _might_
be immediately apparent.... or it might be deliberately obfuscated to
maintain a business model. You should be able to find out quickly.

Basically, if you have the skills to write a replacement device for
updating the DB, then you have the skills to examine the card and
figure out where to start. : )