What is the best way to install & upgrade Cassandra on Ubuntu ?

[Ertio Lew]

How do you install Cassandra on Ubuntu & later how do you upgrade the
installation on the node when an update has arrived ? Do you simply
download & replace the latest tar.gz, untar it to replace the older
cassandra files? How do you do it ? How does this upgrade process differ
for a major version upgrade, like say switching from 1.2 series to 2.0
series ?