You can trim a video in the PosterMyWall editor.
Select the video item, and on the right-side bar, click the trim option.
In the trim window, select the sliders on each end to alter video duration, to cut off specific content from the video.
Moving the slider will change the time on the 'Start Time' and 'End Time'. You can also change start and end time by manually typing in the duration in seconds in the 'Start Time and 'End Time' boxes.
Click the blue 'Trim' button to finalize your changes.