How does one upgrade Redis 2.2 to 2.4?

There are a few PPAs that seems to be tracking the latest redis, this one https://launchpad.net/~chris-lea/+archive/redis-server seems to have builds for most distributions.

You can install from it by running:

sudo add-apt-repository ppa:chris-lea/redis-server
sudo apt-get update
sudo apt-get install redis-server

Edit: https://launchpad.net/~rwky/+archive/redis, now has redis 2.6 and 2.8

As Redis 2.4 is not in the Repository for testing or unstable, you will need to compile it manually. Just uninstall the old version of Redis (via apt), and compile the new one.

Try this to install Redis 2.4

wget http://redis.googlecode.com/files/redis-2.4.0.tar.gz
tar xzvf redis-2.4.0.tar.gz
cd redis-2.4
make install
make test

With the upcoming (slated for October 18th, 2012) release of the next Ubuntu version, Quantal Quetzal the redis-server package is getting upgraded to redis v2.4.15.

It is 3 minor patch versions behind the current stable redis release, but is still likely the best (ie. most supported) way to upgrade from v2.2 to v2.4.