Modify option helps a user to change the SIP investment date, SIP investment amount, SIP Period & frequency of investment. A SIP could only be modified after the completion of 6 installments. For example, if you have registered a monthly SIP which would start from Jan 12, 2017, then you could modify the SIP after 6th installment i.e. only after June 12, 2017.
Please note there are select dates allowed by Mutual Funds on which SIPs get triggered, so while modifying SIP you can select the date of trigger of SIP from the available dates only.
Set Step-Up option allows the user to set-up Step-Up for SIP to automatically increase the SIP investment amount after every period selected by the customer.