Many of us like recording video using iPhone. If you want to share any of the captured clips by emailing to your family and friends or uploading to YouTube, you can actually trim them quickly by using the built-in trim feature in iPhone. It can be done right on your device.
1. From the Photos app, find and select the video you want to trim
2. Grab the handle on the left or right side and move inward, adjust to accommodate the portion of the movie you want to shorten the clip to
3. When finished, tap the yellow Trim button
4. Now you have two options; trim the original video clip down to size, or split the trimmed selection out into a new video clip
Note: It is recommended trimming the video while playing it.
If you want to reduce the size of the video in iPhone camera roll, please refer to: How to reduce iPhone video size