See images below. This is the battery compartment for the MultispeQ Beta. You remove the back cover (sometime requires some force, then you can replace the batteries. If for some reason the battery boxes come lose (as seen below), you can just put them back with the wiring snug between them and the top of the case as seen here. Note the connection to the circuit board - both the black and red wire should still be connected to the board. If one is broken it'll need to be resoldered in place. There is a close-up picture of the solder location of the black and red wire onto the main motherboard.
Battery life is dependent on use, but for typical chlorophyll fluorescence measurements a device without CO2 sensor will last about 1500 measurements, while one with a CO2 sensor will last about 500 measurements (the CO2 sensor hogs power even when not being used).
even more images