Dont trust to "melius2", i think he is just kidding. Of course(!) you may loose ALL(!) your data, as well all your HTML and PHP files as well all Databases. You cannot simply overwrite an older version by a newer, this costs everything!
a) Backup the whole Xampp folder (just for safety reasons).
b) additionally, safe the htdocs folder (all your files and folders)
c) additionally safe the data folder of MySQL
d) optionally safe the configuration files of Apache (httpd.conf etc.) and PHP (php.ini) and MySQL (my.ini or my.cnf), depending on the fact, if you changed some of these.
Then uninstall the old Xampp, install the new one, then restore the htdocs folder (see above), also restore the data folder and if you are quite lucky, the new Xampp will show the old contents now. If anything fails, you finally have your Xampp backup. It might be safe, not to overwrite the existing installation, but create a new one on another PC (or in a VirtualHost environment) instead.