Ubuntu command to download https://launchpad.net/~bitcoin/+archive/bitcoin/+packages What command do I use to get the latest bitcoin package under Ubuntu desktop 12.04 LTS?

Also, what needs to be done to uninstall previous versions?
It's relatively painless 

sudo apt-add-repository add-repository
sudo apt-get update
sudo apt-get install bitcoin-qt


How you get rid of the old copy depends on how it was installed, if it was a default install this will probably get rid of that anyway.