I found ExifTool, which allows you to edit almost any piece of metadata from the command line.
However if you try to edit the shutter speed (using
exiftool file.tif -shutterspeed=1/200), it simply says:
Warning: Sorry, shutterspeed is not writable
Is there any way (using any tool) to change the exposure time in the metadata? Either in a CR2 or TIFF file.