Canon EOS 5D Mark III Instruction Manual page 264

X Editing the Movie's First and Last Scenes
Since the editing is performed in 1-sec. increments (position indicated by

[X]), the exact position where the movie is edited may differ slightly from
the position you specified.
If the card does not have enough free space, [New file] will not be

available.
264
4
Save the movie.
Select [W], then press <0>.

 The save screen will appear.
To save it as a new movie, select

[New file]. To save it and overwrite
the original movie file, select
[Overwrite]. Then press <0>.
On the confirmation screen, select

[OK], then press <0> to save the
edited movie and return to the movie
playback screen.