A simple, quick way to be on the "bleeding edge"
1. Download and install a SVN client. (I use TortoiseSVN located here:
http://tortoisesvn.sourceforge.net/)
2. After install is complete, create a new folder somewhere to download the roundcube files to.
3. Right click on the newly created folder and there should be an option that says SVN Checkout. Choose that.
4. A window should popup called TortoiseSVN: Checkout. In the window where it says URL of repository enter this:
https://svn.roundcube.net/trunk
I left everything else as default.
5. Click okay and it should begin transferring. Click Ok when it completes and you now have the latest available copy.
To update your copy of roundcube, right click on the folder (now it should have a green check mark on the folder icon) and select SVN update. It will the proceed to update your copy.
Experts at this kind of stuff: feel free to flame on and correct me