A: http://download.ushahidi.com
A: If you want to customize the installation heavily, you do need one, specifically one with unicode support. You can find more info here. Or you can use http://crowdmap.com for a hosted version.
A: Download from download.ushahidi.com for the latest release. (or the latest tag from: https://github.com/ushahidi/Ushahidi_Web/tags)
For the current (unrelease) development version checkout code from here: http://github.com/ushahidi/Ushahidi_Web
A: Browse to ./application/config/version.php
A: Follow the instructions given at Ushahidi v2.X Upgrades