How To Change Header CSS

.header {
  position: relative;
  z-index: 10;
  width: 95%;
  margin: auto;
  text-align: center;
}
@media all and (min-width: 1001px) {
 .header {
    width: 980px;
    margin: auto;
  }
}
@media all and (min-width: 1367px) {
/*** HEADER ***/

  .header {
    width: 1200px;
  }
}