How would you produce a higher note on a single string of a stringed instrument?

By pressing you finger down on the string over the fingerboard. This shortens the string, which creates a higher frequency when the string is plucked or played.

So for  string instruments such as violins and violas the closer you’re  fingers are you’re face the higher the pitch. As for Cellos and Bases, the farther you’re hand is from the face makes the pitch higher.