The current Upgrade 1.6.5 is to upgrade 1.6.4, so you will first have to upgrade to 1.6.4 then apply the 1.6.5 upgrade.
Not hard if you follow some simple steps.
From this page find the link to the XAMPP Archive:
Download older versions of XAMPP (even the "old" WAMPP) directly from Source Forge.
Download the 2 Upgrade files you will require:
File Releases-->XAMPP Windows
First thing to remember when upgrading is not to overwrite any of your new configuration files with your older version configuration files - just edit the new configuration files if required with your older edits that you may have done.
Normally configuration files will not be overwritten anyway with new ones when upgrading, unless it is mentioned in any upgrade notes from the developers that configuration files are to be overwritten.
(This situation usually only arises when there is a major XAMPP module upgrade that would require a complete new installation anyway
To start the upgrade process I usually drag my original XAMPP directory in the same directory to create a Copy of XAMPP directory so I have a fall back if things go belly up, and also as a means of having at my disposal the older configuration files so I can effect those edits mentioned above if required.
Then proceed to upgrade from 1.6.3a to 1.6.4 using the method you used to originally install 1.6.3a - use the installer upgrade if you used the installer version to install 1.6.3a and so on.
If asked to overwrite files then select to overwrite your older 1.6.3a files.
Don't do anything else with the 1.6.4 upgrade but when you are satisfied that the upgrade from 1.6.3a to 1.6.4 was effected correctly without any errors showing up then proceed to upgrade to 1.6.5.
When the 1.6.5 upgrade appears to be successful then you can proceed to edit your new configuration files with your previous 1.6.3a configuration edits if you had any.
These upgrades should not effect any databases or anything else that is not effected by upgrading any of the XAMPP modules to their newer versions.
When you are satisfied with your upgrades and XAMPP starts and performs as required then and only then you can delete the Copy of XAMPP you created for your 1.6.3a version - don't try and run both the 1.6.3a version and the 1.6.5 version on the same machine or you may have issues that will give both you and Windows a heavy migraine.
Good luck and using the above method should give you a clean, accurate and trouble free upgrade but as with all things it is not guaranteed in every individual case.
And if all else fails and as a last resort only, just uninstall 1.6.3a then proceed to install the new 1.6.5 version.