The Lot number on your BURST Water Flosser is located behind the water reservoir.
To remove the Water Flosser reservoir, Look at the bottom of your unit. You will see a sliding piece with one lock symbol and one unlock symbol. Ensure the slider is set to unlock.
You can then hold the reservoir in one hand and the neck of the unit in the other and pull it apart, This can be tough the first few times.
If you have difficulty with this then you can place the unit base down on a table or countertop. Then hold the neck of the Water Flosser unit with one hand and rock it 45 degrees toward the power button and with the other hand apply pressure to the reservoir. This should help it unclip and you will be able to view the Lot number!