How to convert the Codeigniter code to normal HTML and PHP? Basically, I want to merge my older Codeigniter code with new HTML, CSS and PHP code.

Not sure this is possible but you leave very few details. Ci has an MVC structure and therefore has many classes that must be included for it to work. Adding html and php in an adhoc fashion will probably lead to breaking functionality or to security issues. That's not to say it can't be done but great care must be taken. Your rewrites for controllers may break for example.