@Lokesh EM

Just import the original controller module class and create a new class that inherit from the original controller class and override the method with the new route and if you don't need to do anything else in the method override just call super.

Like:

from addons.web.controllers.main import Session
from odoo import http

class MySession(Session):
    @http.route('/web/session/change_password', type='json', auth="user") 
    def change_password(self, fields):
         return super(MySession,self).change_password(fields)