The first part of the if statement is

  if(x<380){
    x=x+5;
    y=y;
  }
This will be true if the circle is at the top or bottom of the display which explains why the circle fails to move left along the bottom of the screen. You need 4 tests that identify whether the circle is top, bottom, left or right and to do that each test will need to examine the x and y position.

The following code works check out the if statements.

1. int x = 20;
2. int y = 20;
3. 
   
4. //you must set the variables
5. 
   
6. void setup() {
7.   size(400, 400);
8.   background(#000000);
9. }
10. 
    
11. //setup the interface
12. 
    
13. void draw() {
14.   background(#000000);
15.   ellipse(x, y, 40, 40);
16.   if (x<380 && y <= 20) { //top
17.     x = x + 5;
18.   }
19.   else if (x >= 380 && y <= 380) { // right
20.     y = y + 5;
21.   }
22.   else if (x >= 20  && y >= 380) { // bottom
23.     x = x - 5;
24.   }
25.   else if (x <= 20 && y >= 20) { // left
26.     y = y - 5;
27.   }
28. }

A couple of other things

x = x;

means get the value stored in x and store it in x - i.e. does nothing so you don't need them.
Also
x = x + 5
y = y - 10;

can be written as

x += 5;
y -= 10;

because they dothe same thing