To edit a particular part of a video or audio, use the Split feature:
- On the timeline, tap on the element you wish to edit: video clip / audio track
- Move the playhead (yellow vertical line) to the point where you want to start editing
- Tap Split from the editing toolbar
- Now move the playhead to the point where you want to end editing > Tap Split
- Tap on the fragment you created > Choose from many actions in the toolbar:
you can Delete it or change its Volume, Speed, etc.