It's a microcontroller and not a dedicated LED Display Driver. If you want the functionality to change you will need to reprogram it.
You'll need to reference that datasheet for the STM8S.
You need to determine if the programming lines run out to contacts on the board to make it easy to reprogram. There are two pictures of the back on that page and both are different, but it looks like there might be an unpopulated header for in circuit programming/debugging.
I have not used the STM8 line, so don't take any of this as gospel, but you will need a programmer to program it. The STM8S-Discovery board comes with a ST-Link programmer that will program/debug this chip. Search for STM8S-Discovery and you can buy this pretty cheap.
You then need to actually program it. The code in the chip is likely protected and having the binary isn't going to help you much anyway. In fact if you haven't done any coding before, there is a steep learning curve here for you. You will need to write a program to read the analog input, convert it and then display it on the 7 segment display.
There are definitely people here that have tried out the STM8s line that could give you better detail, but that's generally what you are up against.
Wow. $25 is a huge markup on this thing.