NB: don't forget to stick red wire (thermal sensor?) to the batteries. Ideally to BOTH the batteries, just like it used to be. This is necessary for the safe operation of your device.

Reconnect the pin connectors on the circuit board

Click the battery assembly to the motor housing

At this point you may test whether the electricals are connected correctly (WARNING: make sure nothing is anywhere near the engine blades)

pressing the on/off button initially after the battery replacement shouldn't start the engine, instead the lights will seemingly show the batteries are empty

So, plug a micro-USB charger to your device briefly. This should trick the circuitry into accepting the batteries.

Now, after making sure your fingers are far from the engine blades, when you press the on/off button the engine should come alive.

Don't run the device in this disassembled state for more than ~10 seconds. Remember that running the vacuum without its housing is DANGEROUS