Convert a <span>'s text into a hyperlink

I have a plain text website address inside a <span> tag. I'd like to change that <span> tag into a proper hyperlink with target="_blank"

If you don't want to click through, here is what I have:

<span>http://www.example.com/about</span>

and I need to change that into:

<a href="http://www.example.com/about" target="_blank">http://www.example.com/about</a>