You can edit the financial calculator in single-vehicle.php file (located in autoRoyal theme main folder) from line 1772.
Sorry let me rephrase,
Where can I edit the default interest months. They are set at 3 and 24 respectively, I want to change those numbers.
I'm not sure I follow you here. As you can see on our demo: https://autoroyal.fantasythemes.net/dealership/vehicle/2016-audi-a4-2-0t-premium-4dr-sedan-2-0l-4cyl-turbo-cvt-1847/
You can introduce the rate, months, down payment and the script is updating the data under. There may be a jQuery conflict on your website that is causing the error.
You can send the link to your demo and I will take a look.
For the finance calculator, where can we edit the interest rate and months numbers.