Magazine out and chamber clear, first, please. Double check!
You need to hold down on that takedown latch with one hand, and with the other hand, pull the slide fully to the rear and then lift the rear of the slide up and off the frame, easing it forward and off the front of the fixed barrel. The latch keeps the slide from moving too far to the rear during normal firing; pulling the latch down and out-of-the-way allows the slide a little bit more rearward travel, which gets it to the disassembly point where you can lift it up and pull it off.
Don't pull the trigger or let the hammer slam forward when the slide is off the frame, or damage may result.
Hope this was helpful.