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Remove the old battery pack.
The Batteries are soldered to the circuit Board. You can use your soldering iron here if you like but i think its not really needed. And probably the minus is quite hard to work on because it lays under some other stuff (fig. 3; The circle marks the minus and the square the plus connection).
If you're not using a soldering iron you have to peel off the wire guard from the old batteries to remove them cause they are your connectors for the new batteries. The spring in the bottom cover provides enough tension to keep the battery in place and connected without soldering.