There is no upgrade available from the version you mention.
There have been too many major changes both in components and the methods used to compile the components to have a direct upgrade version(s) available.
You need to backup your own files and folders then uninstall the old version.
Then install the new version and hope that your own files will be compatible with the components of 1.7.7 which can be found in the readme_en.txt file.
Here are some instructions that may help with this procedure:viewtopic.php?p=179699#p179699
Another method that you may prefer is to make sure that you have uninstalled any of the XAMPP components in 1.6.6a running as a Windows service ie remove the ticks in the Svc boxes.
Stop all components and then Exit the control panel - in other words make sure that XAMPP 1.6.6a is completely shut down.
Now simply rename the xampp folder of 1.6.6a to, for example, xampp166a.
Now download the 1.7.7 archive (zip or 7zip) version and extract the xampp folder to the root of a drive or partition and then run the setup_xampp.bat file - same instructions as the link above except you don't need to delete the xampp folder, but as I pointed out, just rename it.
Now you have the best of both worlds 1.6.6a and 1.7.7 but you will only be able to run one at a time making sure that you shut down completely one version before attempting to start up another version.
You would also need to run the setup_xampp.bat file in the old xampp166a folder before starting it again so the XAMPP system paths will be updated with the new folder name - you will have to alter any paths in your own files manually.
Using this method I have several XAMPP versions installed that I use for troubleshooting issues in the forums but I never use the Windows Installer versions, only the zip or 7zip archive versions to make multiple versioning possible.