Open the workout containing the timer you wish to delete

Click the trashcan icon (circled in the picture above)

Click "OK" in the pop-up

You will be asked if you would like to keep the exercises and add them to your unmapped exercise list
-Click "YES" if you would like to be able to keep everything had entered into the timer in order to drag and drop it into a new timer
-Click "NO" if you do not wish to keep the exercises, reps, sets, percents or notes from the timer