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.
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 loose 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.