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 Heat".  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, click on the "Manual Entry" button, change the results as necessary and then save them.