When in the Gallery, tap the video that you want to trim to open the preview. There, at the bottom, you'll see an EDIT button (looks like a Pencil). Tap that and it will take you to the editor. Then just tap the TRIM button (looks like a pair of scissors) and drag the frame sliders that appear on either edge.
If you're editing a SloMo or Super SloMo video, then you should be able to trim the output by playing the video (in SloMo recordings that's where you control the slow motion playback speed and which portions to apply to).
Other options would require you to download a video editing app, and instructions on those would vary per app.