Voultar just posted a great video showing some tips and methods for changing the batteries in your game cartridges. This is something almost all retro gamers will need to do at some point, as well as a great way to practice your modding skills! Just remember to use good tools*!!!
Also, since you already have the cart open, definitely take the time to properly clean them, using isopropyl and a pink eraser…not metal polish or sandpaper:
Modding Tools: https://www.retrorgb.com/tools.html
Cartridge Cleaning: https://www.retrorgb.com/cleangames.html
Amazon Store with more links: https://www.amazon.com/shop/retrorgb
*I’d also like to make the point that “good tools” do not mean “expensive tools”. For years I struggled with certain soldering techniques and no matter how hard I tried, I couldn’t get it right! Then I upgraded to a different station (linked in those pages) that was the same price as my old one, but performed infinitely better. Same with a previous desoldering station: I chose one that was “gun only” to save space and I ended up ruining a few boards with it. Once I upgraded to a large station (that was pretty much the same price) I was able to de-solder like a pro! You don’t need to spend a lot to get good tools, just make sure to buy the right equipment!