The bolt release, which Ruger calls a lifter latch, is that thing on the bottom of the receiver, just forward of the trigger guard. (You should NEVER allow the action of a semi-auto firearm to slam forward, except when you're chambering a round. When chambering a round, AlWAYS let it slam shut from its rearmost position.) Then, ease the slide forward until the bolt is closed.
Before you proceed, put the safety in the safe position. Make sure no red is showing on it. The reason for this is that you do not want the hammer to be suddenly released when you have the trigger assembly out of the gun. That is a good way to damage things, including yourself. So don't do it. Put the safety in the non-firing position.
Now, see the photo. It shows a pin punch pushing the cross pin out of the rear of the receiver. This pin is usually not a particularly tight fit, and can be easily pushed through from either side.
Once the pin is out, move the entire trigger assembly rearward about 1/8", and it should easily come free of the receiver.

