1. Click in the timeline where you want the video excision to begin. (The timeline is just to the right of the word "storyboard.")
Click the small arrow to the right of the SPLIT icon, which is under the preview window. Choose to SPLIT ALL VIDEO TRACKS, or specify the one you want.
2. Click in the timeline where you want the video excision to end and do the same.
3. Select the video clip that was just created - between these two points - and hit the DELETE key, or right-click|DELETE.
Or, if it's easier for you, you can first right-click the video or audio track and choose to UNLINK it. Then right-click the video track and SPLIT SELECTED CLIPS.
Portions of the audio track can be removed in the same manner.
If you make a mistake, press <ctrl-Z> (EDIT|UNDO) immediately to undo the last change. The buffer holds many changes. SAVE AS frequently, incrementing the file # so earlier versions won't be overwritten.