Depends on the code, but in general you could just run setup again.

1. void mousePressed() {
2.    setup();
3. }

You posted in Processing Implementations, what language do you use?
And I wouldn't recommend to recall setup(). No point in calling size() again, to reload files, etc. It is better to create a separate init function that will set the essential varying parameters of the program, to call in setup() and when resetting the application.