DeWalt makes a variety of different models of laser levels. When placed on a flat surface, these levels use a laser technology to determine whether that surface is level or if it needs to be adjusted. If your DeWalt level laser is giving you improper readings (which can be corroborated by using a regular bubble level), it needs to be calibrated. This is something that must be done by DeWalt.
Locate a DeWalt service centre near your location. The DeWalt website has a service centre locator that you can use to find a centre by typing in your Postcode. This will give you the exact address and directions to the nearest service centre.
Take your DeWalt laser level to the nearest service centre and tell a representative that you need to have your unit calibrated. Your unit will be sent to one of the DeWalt centres that performs calibration, at which point the unit will be professionally calibrated and sent back to you. This is covered by your DeWalt warranty, if it is still in effect. The warranty lasts between one and three or more years depending on the model of level laser purchased. Pricing for calibration (if your level is out of warranty) varies depending on model.
If you there is no other service centre near your home, contact one of the DeWalt service centres that performs calibrations and send your unit to them directly. The DeWalt website maintains a list of all the service centres that perform actual laser level calibration. If you can't make it to another service centre, you can contact one of the calibration centres and ship the level to them for service.