Setting up a development installation
Obtaining an image
Downloading an image for the first time
Daily images of Ubuntu development are available on the cdimage server. For a development install, you will want to get either the most recent daily or the current milestone build.
As an example, the latest Ubuntu daily image can always be found at http://cdimage.ubuntu.com/daily-live/current/.
You will get the correct download link for your product (and the milestone, when needed) from the QA Tracker, after you have selected the milestone (daily, Alpha, Beta...) on the left hand side of the product of your choice in the product listing.
Syncing, or updating, a previously downloaded image
If you have already downloaded an image for the current release (eg. Vivid), you can update the image by using rsync or zsync. The commands you need in order to do this can also be found in the download information page.
Once you have an image you can install it into a virtual machine, such as VirtualBox, or onto physical hardware.
Virtual machine installation
When you install a development installation on hardware, it's good to remember that development versions aren't suitable for production machines. When installing, always take backups of important data and be prepared for the situation when the machine doesn't boot correctly. If you can, use a separate hard drive for the testing environment and dual-boot it with your main OS.
The instructions on how to create a bootable USB stick from your downloaded image can be found here.
After installation, you can keep your system updated the same as a typical installation. Use the update manager to update as new packages arrive. Alternatively utilize the command line via sudo apt-get update && sudo apt-get dist-upgrade .
Be very careful that your upgrade is not holding back or removing essential packages. If you encounter an upgrade that looks suspect, it's best to wait and try again at a later time. The archive state in general should be kept clean, but a partial upgrade is possible.