Because your installation would appear to be very old then it would not matter how it was installed in the first place as you need to uninstall completely your old version to upgrade to the latest 1.6.8.
In the read-en.txt file and on the XAMPP For Windows web site are some handy tips on how to do this and, of course, on these forums that a search will reveal - even posted here within the last week or so may help.
Basically you need to stop all XAMPP components and uninstall any that you have running as a service not forgetting the Control Panel.
Then make a backup of all configuration files you edited and any other files needed, then delete the XAMPP folder in total - if there is an uninstall exe file then that may be an indication you originally used the install version but I have never used the install version as outlined below, so I do not know the exact name of the uninstall file.
I recommend, if you are doing a clean uninstall/install using either the 7Zip or the Zip methods as, for me, I have more control over how and where it will be installed and the same control for future upgrades.