Locate a DeWalt service center near your location. The DeWalt website has a service center locator that you can use to find a center by typing in your ZIP code. This will give you the exact address and directions to the nearest service center.
Take your DeWalt laser level to the nearest service center and tell a representative that you need to have your unit calibrated. Your unit will be sent to one of the DeWalt centers 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 center near your home, contact one of the DeWalt service centers that performs calibrations and send your unit to them directly. The DeWalt website maintains a list of all the service centers that perform actual laser level calibration. If you can't make it to another service center, you can contact one of the calibration centers and ship the level to them for service.