Update old joomla site to latest joomla version

We have an old joomla site that is active in an IP address, however we have shut down the server, due to the high vulnerability it has. Its running under: PHP Version 5.3.9 Windows NT HWC-HWP-3509510 6.2 build 9200 (Unknow Windows version Standard Edition) i586 Database Version 5.5.60-log Database Collation utf8_general_ci PHP Version 5.3.9 Web Server Apache/2.4.34 (Win64) mod_fcgid/2.3.9 WebServer to PHP Interface cgi-fcgi Joomla! Version Joomla! 2.5.11 Stable [ Ember ] 26-April-2013 14:00 GMT Joomla! Platform Version Joomla Platform 11.4.0 Stable [ Brian Kernighan ] 03-Jan-2012 00:00 GMT User Agent Mozilla/5.0 (Windows NT 6.1; rv:62.0) Gecko/20100101 Firefox/62.0 Makes use of the Zend Scripting Language Engine: Zend Engine v2.3.0, with the ionCube PHP Loader v10.2.0, with Zend Guard Loader v3.3, We would like to Update this old joomla site to latest joomla version and be sure of its total functionality in a perfect way We have been advice to do a new site, however the site we have is a complex one, we went through the whole process of creating it once, and we do not want to go over it again, nor have the resources to do so. We need a fast simple way to actualize the site we have. It´s also difficult to just buy a Real Estate theme and adapt it as it has many complex functionalities that were done specially for this site when required. Thanks