Select Host a Meeting on the left navigation bar to expand the menu and click the My Recorded Meetings link.
For the recording you wish to modify the start and end time for playback - click the button with three(3) dots on the far right side.
NOTE: Be sure that your recording is listed as an ARF format in your list of recordings — it is not possible to edit MP4 recordings in the web interface.
Select Modify from the popup list.
Near the middle of the options list you will find the option to trim the start and end time of the video. This is a non-destructive edit - meaning you will not actually lose the content rather you are telling WebEx where to start and end the playback. You can perform this action even after you have shared a link to the recording with your audience. The next time your audience watches the recording these trim setting will apply.