[NSIS] Update installer

Hi everyone!
I need to make an installer that must be simply an update.
So it must locate the installation folder of the software and sobstitutes some files.
How can I locate the installation folder of the software ?
Thanks in advance

Well, I noticed that if i put in the installer file the following command (to write the installation path in windows registry)

WriteRegStr HKCU "Software\MYSOFTWARE" "" $INSTDIR

In an update installer I can recall it with

ReadRegStr $INSTDIR HKCU "Software\MYSOFTWARE" ""

so that in INSTDIR variable I have the path of the software.

If there is any other better solution please let me know.
Thanks again
-Fulk-