It is good for you to have chosen costa del mar sunglasses, because that type of sunglasses always provide good feelings and design to you. As to your question, you should look something through the product manual, maybe on it you could see how to adjust the sunglasses. But if you do not succeed, maybe you should ask for a change. Anyway, we need something suitable to us even if it means paying more.
If your sunglasses frames slide out of place, it is necessary to adjust it. You can take the following steps. If the sunglasses always slip down your nose, the distance between nose pads could be too far. As for it, you can pinch them to be closer. Also, it may caused by that the earpieces are too loose, you should twist the earpieces towards the head.
If the sunglasses always slip down your nose when you wear it, it may mean that the sunglasses may be a little bigger for you. You'd better adjust the position of the nose pad of the sunglasses which could be tight and let you wear comfortable. You could use the screwdriver to do this.