The main problem in your code is the global boolean INSIDE. If you place it inside the class it'll work better.

But there are also a lot of other possible improvements, see below.

Adapted Code

1. Button B1 = new Button (5, 5, 200, 50);
2. Button B2 = new Button (5, 60, 200, 50);
3.  
4. void setup() {
5.   size (500, 500);
6. }
7.  
8. void draw() {
9.   background (0);
10.   B1.display();
11.   B2.display();
12. }
13.  
14. class Button {
15.   int x, y, w, h;
16.  
17.   Button (int x, int y, int w, int h) {
18.     this.x = x;
19.     this.y = y;
20.     this.w = w;
21.     this.h = h;
22.   }
23.  
24.   void display() {
25.     if (mouseX >= x && mouseX <= x+w && mouseY >= y && mouseY <= y+h) {
26.       fill(255, 0, 0);
27.     } else {
28.       fill(255);
29.     }
30.     rect (x, y, w, h);
31.   }
32. }

For a better organization, it'd be nice to have a separate method to deal w/ hovering check:

final class Button {
  final short x, y, w, h;

  Button (int xx, int yy, int ww, int hh) {
    x = (short) xx;
    y = (short) yy;
    w = (short) ww;
    h = (short) hh;
  }

  void display() {
    fill ( isHovering() ? #FF0000 : -1 ); 
    rect (x, y, w, h);
  }

  boolean isHovering() {
    return mouseX >= x & mouseX < x+w & 
      mouseY >= y & mouseY < y+h;
  }
}

@ GoToLoop I have seen that kind of if statement in  fill ( isHovering() ? #FF0000 : -1 );   but truthfully I have never used it

Nice you've liked it! 
If it's another example, a different post would be more appropriate me guess.

I  figured out the changes ,and i realised it looks a bit like what i had before , but i did something wrong whit my imageMode, now that this is resolved , i would like to ask an other question, if i wanted to make it so that if one clicks on a rectangle something else would happen , how would i proceed to make something eles happen for each single rectangle?

What you put in one single class, will work for all of it's object instances. If you want different behaviors you can incorporate multiple in the class and for example let each object choose randomly from them. Or you could use inheritance to create a base class (animal) and multiple subclasses with different behaviors (dog, cat, mouse). It depends on what you want to do. For now, the first option is probably a good start.