For those of you (like me) that have slow net connections, I thought I'd mention:
Once you have a copy of MultiBit on your machine, to upgrade to the latest version you only need to update the
multibit-exe.jar in your MultiBit installation directory. This is only a third the size of the full installer so I have started to upload it into the multibit.org releases directory:
https://multibit.org/releases/On Windows I wrap the executable jar file into an exe as people are more familiar with those but you can still use the multibit-exe.jar on Windows too.
To run it you can normally just double click it. Or if your operating system is not set up to run jars, just open a command line, 'cd' to the directory and type:
> java -jar multibit-exe.jar
(Or write a shell script/ batch file to do the same).
As this involves knowing a little about what is going on 'under the hood' I will leave it off the regular installation notes on the multibit.org site.
If you are wondering, the other two thirds of the installer is the (headers only) blockchain. If you have already installed MultiBit you've already got this so it's not strictly necessary to have it again. (It is used as a fallback should the user copy of the blockchain get damaged for any reason so the re-download is not wasted).