8 votesGustavo commented
1) Export the full presentation ("Export ZIP" option);
2) Unpack the zip file
3) Open the index.html file for edition (in Notepad or other simple text editing software; do not use Word or other complex tool)
4) At the end of the file, find a "<script>" tag that contains the "Reveal.initialize" text
5) The "Reveal.initialize" is a call that contains many parameters, spanning many lines. This call ends at a line with "});"
6) After this "});", insert the following code below:
40: 'next', // down arrow key mapped to next slide
38: 'prev', // up arrow key mapped to previous slide
7) Save and close
8) Open index.html in Chrome or other browser, and test it.