The cart is empty

Normally, to start a heat, you would click on the "Ready Timer" button.  For a heat that has already been run, that button will instead say "Rerun".  Just click on that button to rerun the heat.  You will get a warning asking if you want to reset the times, and then you will run your heat as before.

If you are inputting results manually, just make the necessary changes before you save them.  If you had already saved the changes, you can delete out the incorrect run and add a new run.