change colour of all letters

A book I'm reading has set all the letters throughout the book in the colour red, which make it annoying to read in my kindle, as the contrast is bad.
How do I change all the red letters to black letters ?
The format is Mobi.
If the answer is that I have to use the Calibre Editor, I have to confess that I am a complete novice regarding the editor :)

Thanks from
Morten Levy, Denmark