My b2evolution Version: 1.10.x

What this plugin will do is so you can add your own things to render. So:

So we go to it in the tools tab, and add a field called "Download URL", with the code "Download_URL". Set the formatting for it to "<a href="%Download_URL%" title="Download" class="download">Downlod</a>". And then in our post, we will see the custom renderer button, click it, see a Download URL field, we add our url there, click attach, and the renderer code is added to the post.

So that way, when you change your mind about how you want your download links to look, there is one place you change it :)