Only videos that have not been uploaded can be edited on the iOS app.
If the video has already been uploaded you can edit the file on the computer.
Written Guide
Open the Service.
Tap Upload
If you don't see your video, tap "+" in the top right corner to add it from your Camera Roll.
Tap Trim Video
Tap and Drag to set In and Out. It will turn orange to signify you are setting the Video In and Out points.
Tap Save.
After it's done compressing tap Upload and wait for the Completed Message.
Note: Do not change networks during the upload. This will cause the upload to fail. Large videos may take time to upload. Upload times vary depending on internet upload strength.