the problem is that you reset /recreate laserXpo laserYpos at the beginning of fire in every frame. So even if you try to count laserYpos down by 10 you reset it the nexxt frame, so nothing ever happens.
you have write it like this somehow.

what i also did is, just reset it on mousePressed
that means you can fire a new shot with the mouse

1. Turrent myTurrent;
   
   int bg = 255;
   void setup() {
     size(250,400);
     background(bg);
   
     myTurrent = new Turrent(125,200);
   }
   
   void draw() {
     background(255);
     myTurrent.display();
     myTurrent.fire();
   }
   
   class Turrent {
     float xpos;
     float ypos;
     float laserXpos ;
     float laserYpos  ;
   
     Turrent(float tmpXpos, float tmpYpos) {
       xpos = tmpXpos;
       ypos = tmpYpos;
       laserXpos = xpos;
       laserYpos = ypos;
     }
   
     void display() {
       stroke(0);
       fill(150);
       ellipseMode(CENTER);
       ellipse(xpos, ypos, 30,40);
   
       fill(0);
       rect(xpos - 6, ypos - 30, 3, 20);
       rect(xpos + 3, ypos - 30, 3, 20);
     }
   
     void fire() {
   
       stroke(255,0,0);
       line(xpos - 5, laserYpos - 50, xpos - 5, laserYpos - 100);
       line(xpos + 3, laserYpos - 50, xpos + 3, laserYpos - 100);
       laserYpos = laserYpos - 10 ;
       if(mousePressed){
         laserXpos = xpos;
         laserYpos = ypos;
       }
   
     }
   
   }