How to change the default width of container in MaterializeCSS?

Just override the property by making sure that your style is read by the browser after linking to, or importing materialize.css

You would need to copy and change these values:

.container {
  margin: 0 auto;
  max-width: 1280px;
  width: 90%;
}
@media only screen and (min-width: 601px) {
  .container {
    width: 85%;
  }
}
@media only screen and (min-width: 993px) {
  .container {
    width: 70%;
  }
}