Install a periscope dryer vent duct by removing the existing plastic tube vent and replacing it with a vertical metal vent. A screwdriver and vacuum cleaner are necessary tools for this job.
Unplug the dryer, and if it's a gas dryer, shut off the gas feed valve. Move the dryer away from the wall to gain access to the back. Remove the round plastic vent by unscrewing the bolt on the clamp that holds it to the dryer exhaust outlet at one end and the external duct at the other. Vacuum out any lint in the dryer and the duct.
Use a metal band clamp to secure the lower end of the periscope vent to the exhaust opening on the back of the dryer. Gently push the new vent onto the exhaust opening. Then, tighten the clamp. Extend the top of the periscope vent up until it's a couple inches above the dryer. Plug the dryer in, and if necessary, restore the gas flow.
Return the dryer to its position, and attach a flexible elbow to the top of the periscope vent using a band clamp. Attach another flexible elbow to the external duct with a band clamp, and then stretch each tube until they meet. The two ends have special fittings that snap together, so no clamp is necessary to connect them.