Avidemux (a basic, free video editor) will do it. Look in the "Filters" options under "Video Output". You can do a horizontal or vertical flip, or rotate by 90/180/270 degrees.
Avidemux - Downloads
or a portable version if you don't want to install it:
Avidemux Portable | PortableApps.com - Portable software for USB, portable and cloud drives
Instructions here:
build:doctop [Avidemux]
TFP