I need to change the rel=canonical on a single post, so that the link points to a URL on a different domain. The reason for this is that I have duplicate content on 2 websites, and I would like one of them to be the 'original' source for SEO purposes.

I think I need to add something to functions.php to select the page number and edit wpseo_canonical, but everything I have tried so far breaks my site. Any suggestions would be much appreciated!