First, UNLOAD the gun.
Remove the grips
Next, inside the frame, at the rear of the trigger guard, there is a pin you need to depress. When you depress it, you can pull the whole trigger group down & out. Be careful, and look at how the part assy comes out. They have to go back in the same way, and once removed from the frame, a few can fall off the assy.
Once that assy is removed, you can open the cylinder, swing it out, and pull it out forward.
The job will be done easier if you remove the hammer spring/strut assy, the hammer pivot,, the hammer, all before you try to remove the trigger group assy. It allows the easier access to the pin that locks the trigger group to the frame.