Coloured lines on either top or bottom screens of the Nintendo DS can occur when the console is dropped or if it's too cold. Removing the battery from the unit will reset everything back to the original factory settings. Determining if the unit is too cold or in need of a hard reset is the simplest course of action. If neither of these conditions is the cause of the problem, attempting the necessary repair is difficult and you might void the warranty.
Power off the Nintendo DS.
Warm up Nintendo DS if you've left it outside or in the car for a while. Let the unit come to room temperature.
Remove the battery from the underside of the Nintendo DS using the small screwdriver.
Write down the serial number of the Nintendo DS located on the underside of the unit, this includes the last number in the square box. This serial number usually starts with a UG (Nintendo DS Lite), NU (Original Nintendo DS), or TW (Nintendo DSi). Keep this in the event that the lines on the screen persist.
Insert the battery into the Nintendo DS unit. Check the battery is inserted in the correct position and hand tighten the screw. Do not over tighten.
Power on the Nintendo DS.
Lines that fade away after powering the system off is normal and is not considered a repair issue. Lines that continue to display usually indicate a damaged unit -- use the serial number to get an estimate of the repair cost. Nintendo offers a one-year warranty for all new systems.
The one-year warranty on a new Nintendo DS will be voided if the internal components of the unit have been tampered with or if any "do it yourself" type of repair has been attempted.