... and the other side

After the existing code:

  if (px > 40) {
    dpx = -dpx;
  }
    

insert some similar code:

  if (px < -40) {
    dpx = -dpx;
  }
    

Both of the above two sections of code together
can be replaced by the single following section:

  if ((px < -40) || (px > 40)) {
    dpx = -dpx;
  }
    

Although compact, this code is more difficult to read.
The || notation represents the logical OR.
Note also the need for an extra set of parentheses.