You'll need a 50mm Phillips screwdriver and a flathead screwdriver as thin as possible.
Place the Waka Waka with the yellow side up. Flip the standard just as in the right picture.
First you remove the two yellow rubbers using your nail or the screwdriver. Than use the Phillips screwdriver to remove the two screws.
Turn the Waka Waka upside down. Make sure the Waka Waka is placed on a flat surface. Place the flathead screwdriver between the yellow and the black part as can be seen in the second picture. Force will be needed. If one side came loose, do the same for the other side.
Now you see all the components in the Waka Waka. The battery is attached to the case with tape, so you have to use force to pull it off.
Remove the battery from the PCB by pulling the cables out of the clicking system.
While placing the new battery, make sure the connector is not upside down as you can see in the first picture. Click it in the PCB.
Use double sided tape to attache the battery to the case. Place the battery back in the case as shown in the second picture.
Push the electricity cables aside. Place the springs in the case next to the battery.
Place the standard. Make sure the standard is placed in the right position.
Place the yellow part above the other half. First push where the springs are placed. Use force. Than push the rest until you hear a click.
Replace the screws and the yellow plugs. Check if everything is working again.