The recoil spring assembly is on the bottom of the barrel. It is held in place by the spring's own tension against a notch in the barrel (2nd picture) and its slot in the end of the slide.

While holding the slide with the muzzle pointing away from you, put your thumb or index finger on the rear-facing portion of the recoil spring assembly. Push the assembly forward to free it from the tension against the barrel, then release the tension and pull it out of its slot in the slide.