It's not your first time updating First Strike, believe it or not .

In order to play First Strike on a server for the past months, you had to update First Strike 13 separate times.  The First Strike launcher doubles as an autopatcher, and was actually used by many to update to 1.4 from 1.3.  There have been 13 mini-patches or "builds" released since 1.4 came out, which the launcher downloads and installs automatically when they come out.   I can't confirm (but it doesn't make sense not to have this), but there should be a build released which will contain the new 1.5 content, which your launcher will download and install when you try to launch First Strike after playing the game.

Alternatively, there will be a standalone installer for First Strike 1.5 released which you can download and use to install 1.5.  This will be hosted in a number of different locations there shouldn't be any issues in finding a fast server. 

Will I have to uninstall 1.4 and clear the cache directory or just run this new patch on top of my existing install?