Here's another way that I found to replace the battery, and it is without soldering.

I didn't play my gold or silver versions in over a year, but today I decided to take a look. To my horror, my games were deleted, and when I restarted, I couldn't save!

So I took a look at this post, and fixed the battery (short term, at least. I'll see in a few weeks if it works over a long period of time).

Here's the supplies I used:

Pair of tweezers
Needlenose pliers
Swiss Army Knife or any reliable, smooth blade
1 CR2032 20mm battery (or CR2025 battery)
Electrical tape

Here's how I did it:

Use the tweezers to unscrew the game cover off (this might be the hardest part). Slide the cover down, and gently pull it off.

Find the round battery in the upper right-hand side of the game. Notice that the negative tab is fastened to the top. GENTLY use the knife to detach the two attachment points of the tab to the battery. Be sure to NOT break or distort the tab beyond repair.

Now, the battery should be fastened underneath by a "+" tab. Lift the battery and the tab up so that they are still attached, but you can see the underside. Be careful not to rip or break the tab.

Now, use the knife again to detach the two points of attachment. The battery should be free now. Using the pliers, gently straighten the positive and negative tabs, but make sure the negative one keeps its original "bend".

Notice that there is a plastic ring around the removed battery. Use the knife to carefully slice this ring, and fasten it around the new battery, using electrical tape to secure it.

Flatten the positive tab and place the replacement battery on top (make sure the bigger side faces down). Now, place the negative tab on top of the battery.

Press gently down on the battery and tabs, and use electrical tape to wrap around the circuitboard, holding the battery and tabs in place. Be especially careful with the white contraption in the upper left of the board, since it controls the time element in the game.

Put the cover back on, screw the bolt back on, and hope that it worked!