You cannot. You can only speed it up or slow it down. The dashed lines indicate how to do so. Black dashed is click and hold, green dash is click, click hold.
The diagram says brighter/faster and dimmer/slower for the same action, ergo you can only change one of those depending on what mode it is.
The tactical strobe appears to only work at max (or very close to max) output. It gets very disorienting at certain speeds, I would play around with that if you don’t find it particularly effective out of the box.
ETA: I have tested this with a LT1, KR4, FW3A, SC31 Pro, and D4v2.
The Lightpainter flashlight from Light Painting Paradise is the only flashlight I’m aware of where both strobe brightness and frequency can be adjusted.