For more information about the "this" keyword in JavaScript, I encourage you to have a look at the questions that deal about this specific problem, such as this one : How does the "this" keyword work?
However, from the Phaser point of view, the anchor point is the anchor of Phaser Sprite or a Phaser Image, so to change the anchor point of any sprite, change it after the sprite has been created :
var sprite = game.add.sprite(10,10,'');
// and then
sprite.anchor.set(0.5);
Also your example was not super clear, "this.assets" isn't super explicit in my opinion as a sprite doesn't necessarily come from an asset, which example were you looking at ?