If you're careful and assuming the legs aren't all corroded, you shouldn't be breaking any legs on the battery. Simply heat up your soldering iron (make sure you've got a nice sharp tip), sneak a small flat blad screwdriver under the battery (or just use your fingers), apply the iron to the end with only one leg and carefuly lift that end of the battery up and out. Using the iron again, heat up one of the remaining two legs on the other end and lift her up a little, but don't fully remove. Repeat for the other side until the leg comes fully out of the board, then tackle the remaining leg. IF there are four legs on the battery, I'd take some de-soldering wick to 'em and try to remove as much solder as you can that way so the battery can be gently wriggled/pried out.
If you end up breaking a leg off though, it can usually be removed with a fine needle-nosed pliars while heating up the solder joint. No big whoop!
