There is a takedown 'button' on the frame, just forward of the trigger guard, and underneath the slide. Different folks do this different ways, in terms of hand placement, but I'd suggest this method:
UNLOAD GUN (so you don't sue me for not reminding you, if anything goes wrong. )
Drop magazine
Decock
Thumb and forefinger on back of slide, heel of hand resting on backstrap. Almost as though you were holding it to shoot, but fingers are higher up, grasping slide.
Thumb and forevinger over top of slide, grasping takedown lever, slide downward toward guard, as you pull slightly backward on slide to relieve tension on the locking pin inside.
Slide will release, continue motion all the way forward to clear rails.
Once you see how it works, you can repeat this, and take down the gun in about 5 seconds flat.