Nintendo was serious about not letting people easily disassemble their hardware! To open your cartridge, you'll need a 3.8mm security screwdriver. These are inexpensive and will let you access a wide variety of Nintendo games and systems.
Remove the single screw from the back of the cartridge and remove the circuit board from its plastic housing.
The Game Boy game battery is soldered to the circuit board via two tabs. Heat up each tab and then use your solder sucker to remove the old solder.
If you haven't used a solder sucker before, it's easy -- just press the plunger down, heat up the solder with your soldering iron, and then press the button on the solder sucker and the plunger will suck up the melted solder.
If you don't have a solder sucker, you can also use a slightly less expensive desoldering wick -- but I find these much harder to use.
Using soldering "helping hands" can help to hold the circuit board but aren't necessary.