Yes, you as a retail investor can determine the upper limit of your SIP top-up. While setting the step-up instructions, you can specify the maximum amount to which the SIP can be stepped up, endowing you control over your increased investment and ensuring it is in line with your financial potential.

The frequency of the step-up in an SIP is usually annual. This infers that you can choose to enhance your SIP contribution once every year by a predefined percentage or amount.

The amount to be stepped up in an SIP is determined by you and can differ based on your financial goals and capacity. It can be a fixed amount or a specific percentage of the initial SIP investment. 

Yes, as an investor, you can modify the top-up facility that you have already availed. Most mutual fund houses provide the flexibility to change or cancel the top-up instructions according to your needs. However, it is recommended to check the modification conditions of specific mutual fund schemes beforehand.