How to Add Timings and Narration to a PowerPoint 2013 Presentation
If you messed up on any slide, select that slide, click Record Slide Show, and then choose Start Recording from Current Slide.
Then re-record the timings and narration for that slide. Press Esc to stop recording.
You can re-record more than one slide in this way; just press Enter or click the mouse to advance through all the slides you want to re-record. Press Esc to stop recording.