You can use QuickTime Player 
to trim your video files. To Trim a Video, use the following steps:
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In QuickTime Player
, select Edit > Trim, so that the trimming bar appears.
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Use the Trimming Bar to select the portion of the movie that you want to keep. Dragging the highlighted areas to make your selection.
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Click the Trim button and the movie will be shortened to the portion you selected.
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Don't forget to Export your movie to your desired location. To do this go to File > Export and select either 1080p (HD), 720p or 480p (SD).