Welcome
In your pic.
1. to do a change you should be at less than 5mph.
2. move the stick to low/high.
3. off you go.
I usually find that I will put it in Low at standstill drive out of the situation then stop and go back to high. we own a defender but its pretty much the same.
Welcome to the forum.
If you do not have an owner's manual, try to get one. It will explain what it is and how it works.
Broadly the Lo position in the transfer box changes the ratio of output from the gear box to the prop-shafts and then the wheels. To a position where the engine turns the wheels at a lower speed but with more power. The gear box acts as usual.
Things are just a little more interesting if you have an automatic gearbox.