How do I close cfprefsd so that I can update Flash on mavericks
There are actually two cfprefsd processes running. One is the daemon owned by root and the agent owned by the logged in user.

If you kill both then the agent will respawn, notice the missing daemon and get it respawned.

If you just kill the daemon it takes a while to respawn so you should be able to run the updater.

At the command line :-

ps ax | grep cfprefsd

which will list the two processes with the process ID in the left column - 

7308   ??  S      0:00.05 /usr/sbin/cfprefsd agent
7365   ??  Ss     0:00.02 /usr/sbin/cfprefsd daemon


so then 

sudo kill -9 7365 and enter your password when prompted.

or whatever the PID is and quickly run the updater.

Why Adobe thinks it needs to have this process out of the way is a question for another day but Adobe updaters are known for their quirks.