For a start, you need to unclip it from the roof. - They lock down, and can take a bit of force to unclip.. Look carefully and you will see the top section (the bow) has a lug which clips into the roof section. One clip forwards, one clip backwards.
Then it depends whether there are motors fitted to the panto's, or not..
If there are, and you are on analogue, then the rear panto will go up as the loco starts to move.
If the loco is fitted with an original decoder, then there are two modes of pantograph operation.. In one mode F3 will cycle through (from both down) one up, two up, first down, second down for each press of F3. Second mode needs activating by pressing F7.. Then F3 toggles one panto up and down, and F4 the other.
**Note: All the above from memory.