It might sound easy but it is not.
The thing is that you can easily change container width like this
.container {
width: 1280px;
}
But sidebar will get bigger as well, you will need to change WordPress content width which is defined in functions.php, also you will have to change size featured image size and regenerate thumbnails. Also you will lose some of the responsiveness on various devices.
One option, which is a bit easier, is to make sidebar smaller to leave more space for content are but then again, content, width, featured image size, and thumbnails will have to be regenerated.