I just finished the silicone work.
Some unused silicone were stuck in the nozzle,how can I clean it and use the nozzle again?

A lot of people put a screw into the back side of the nozzle to pull out the cured silicone.

Personally, I use a small oral syringe used to give kids medicine for application. I actually stick the syringe INTO the opening of the tube, draw in some silicone, then apply that to the groove. My mess and waste is minimized.