You must be in mode 3 to do this. To change the mode, press and hold the button for more than 10 seconds and wait until mode 1=1x flashing, (13 seconds for) mode 2=2x flashing, (16 seconds for) 3=3x flashing is signalled. Release.
In mode 3, switch on and hold for 2 seconds. In modes 1 and 2, the sequences of the brightness levels that can be selected by pressing briefly are different:
Mode 1 ON = bright - medium - low
Mode 2 ON = low - medium - bright
Mode 3 ON = (analogue 1) plus strobe function by pressing a little longer.
The good man here explains this very well (from 5' he starts with the modes:
https://youtu.be/zZDiUgU5Yi...