Select the clip, not the page
Open the design and click the footage you want to shorten. If text, music or other elements sit over it, confirm the video itself is selected before adjusting its duration. Play near the point where the useful action begins or ends. Cutting the wrong element can change the page without changing the footage you intended to fix.
PATH A
Use an edge trim for an unwanted opening or ending, not for a passage surrounded by footage you want to keep.