Normally, you would have to destroy the trigger and then install a new one. Unlike the larger guns there is no hole in the top of the trigger to drive the pin down and out. You could try to guess wheres the hole would be and drill one of your own there or try to pull the pin out in some way.
You really shouldn't need to replace it unless there is an issue. Like PshootR posted you have to break the trigger to remove the pin. I had to replace mine but it had slipped out of place and I couldn't get it back. As long as you don't over stretch it when you remove the frame it should be fine for thousands of rounds.