You could install a new version using these instructions:viewtopic.php?p=179699#p179699
But recommend you simply rename your existing xampp folder, for example xampp_old (or xampp153a) rather than uninstall the older version then extract the new version zip file to the C:\ drive or drive of choice - you must stop and uninstall any services you may have running by removing the ticks in any Svc boxes and making sure that Apache and MySQL are not running and then you can Exit the control panel, in other words your 1.5.3a version must be completely stopped before you can start the new version's components.
You can then copy over your own files and folders from the htdocs folder of your old version to the htdocs folder of your new version - but do not copy over any files or folders of the older version xampp system as they would not be compatible with the new version.
You should first backup any databases you have in the old xampp version using phpMyAdmin to an sql file so you can import them after you have established that your new version is working properly and this also applies before copying your own files, first go to http://localhost
and after setting your language and before anything else make sure all the demos work in the Welcome page as outlined in the above install instructions link.