To update to v5, you can either download the Eventide Direct Updater from the Space support page (http://www.eventide.com/AudioDivision/Support/Stompboxes/Space.aspx) or you can download H9 Control from the H9 support page (http://www.eventide.com/AudioDivision/Support/Stompboxes/H9.aspx). If you launch either of those apps with your Space connected, they'll take care of downloading and installing v5.
If you're having trouble changing the expression pedal mapping, I'd recommend hooking up your Space to H9 Control. It will show what's currently mapped to the expression pedal (and for Nebula it is sized), and it makes it easy to remove mappings and add new ones. The manual explains how in the "Programming the Expression Pedal Patch" section: