The trigger on a Ryobi trimmer runs from the front handle to the carburettor via a metal cable. This trigger, when squeezed, adjusts the flow of fuel mixing in the carburettor, increasing the speed of your engine. The cable can come loose, the plastic trigger can break or otherwise come apart so you will need to replace it. If you notice the trigger locking or sticking you will need to replace it, otherwise you won't be able to control the speed of your engine.
Switch the Ryobi's engine off. Unscrew the six screws that hold the trigger cover in place over the trigger. Pop the two halves of the trigger cover open with the flat tip of the screwdriver. Pry the halves off the shaft.
Unscrew the trigger spring's housing screw with your fingers. Disconnect the spring from the trigger. Pull the throttle cable away from the trigger. Take the trigger off the shaft.
Set the new trigger in place on the shaft. Reattach the throttle cable to the end of the trigger. Insert the spring and tighten the housing screw with your fingers. Place the two halves around the trigger.
Set the trigger and two halves around the shaft of the trimmer. Tighten down all six screws to secure the trigger and throttle in place. Squeeze the trigger to make sure it functions properly before restarting the trimmer.