Once you have finished your recording, you can clean things up (pauses, coughing, etc.) by using the selector tool in the upper left-hand corner (looks like a cursor). Simply click and drag to highlight the area you want to remove and then press the delete key.
To undo something, go to Edit > Undo or press <Ctrl>Z.
To make precise edits, you might want to zoom in using the magnifying glass tool in the upper-left hand corner.