If you want to cut out the beginning or end of a webinar, you can do this with the trimming feature.

After the webinar is over, head over to /replays. On top of the video preview, you'll find a trim button, click on the button.

In this screenshot, you see I trimmed out the beginning of this webinar. The section highlighted in red is what remains as the replay.

Click save and exit to save. The trimmed video is automatically set as the replay.