First of all be sure that you really want to keep this keyboard, given the low prices.
After removing the screws, the two half shells are usually snapped together. You would normally be able to pry them off with a rounded knive of a flat screw driver.
If this is only to clean dirt on the keyboard, then the shell side with the keys can be washed in water with soap and a brush, as usually here is no circuitry in this part. Rinse thouroughly and use clean compressed air to expel the water. Let dry open for a coupe of days.
If you are a curious person, you may also want to clean the rubber matte that provides the contacts and the spring action for the keys. It can be done but be very careful. Besides liquids, keys that don't work are caused by dust or hairs between the rubber pads and the printed circuit board. I have cleaned my trusty keytronics successfully, but I am very well equipped for repairing electronics.