I have a 1996 chevy with the 4.3L vortec engine and I believe that the timing has jumped. Is there a way that I can manually reset the timing myself instead of putting it in the repair shop?

No. You will notice that the plate on the distributor is not like the traditional old school distributors with a slot to adjust the timing, it simply has a bolt hole (i.e. non adjustable). The timing is controlled by the ECU. The only thing that can change your timing is for your timing chain to have jumped a tooth on your timing sprocket. In short the timing chain is worn out and needs to be changed immediately. I can't remember if the Vortec is an interference engine or not, but if it is and you jump the timing chain far enough or break the chain you are in for a world of hurt when the valves go through your piston crowns. Once you change the chain according to the factory manual that will have set the static timing to factory standards and the ECU takes over from there advancing and retarding as needed while the engine is running.