This isn't worth doing as there is no benefit, the current URLs are already search engine friendly.
However if you really want to, there are 2 main steps:
1. Override the qa_q_request function. The last line returns the actual URL, you'll need to change that to move the number to the end. Override docs here: http://docs.question2answer.org/plugins/overrides/
2. Add some lines to the beginning of your htaccess file that rewrite your new links to the old format.