I have noticed since updating to iOS7 that Apple have not made it easy to figure out how you can edit/trim a video without another app but there is a way without the need for another app.
First goto your Photos App and click once into the video you wish to edit.
You now want to hold your finger in the location the red circle has highlighted like so:
If you are holding your finger in the right place the video timeline should now change to yellow like this:
You can now trim your video to the bit you want by simply holding down your finger on the left or right yellow arrows and moving them to the selection of the video you want.
When your finished and your ready to save your selection, simply hit the “Trim” button up the top right
You should now have a little popup at the bottom of your phone prompting you with two options
You can now simply choose to either save the selection as a new clip or to overwrite the existing video. I recommend saving as a new video file to avoid overwriting the original.