Upgrade Web Site to PHP Version 8.5

My site, https://gbrainsinfotech.com/, is still running on PHP 7.3. I want the entire installation — core files, any themes, and every plugin or custom snippet — migrated to PHP 8.5 with zero loss of functionality and no visible downtime... (Budget: ₹1500 - ₹12500 INR, Jobs: Database Management, GoDaddy, HTML, MySQL, PHP, Web Development, Web Hosting, WordPress)