Here's how to do it:
Here's a short video to guide you through the steps:
Click on the Dashboard --> Goal in which you have SIP running --> SIP Centre --> SIP Actions --> Change SIP date.
Please note that you would be able to set up a SIP from your dashboard up to 1 day prior to the scheduled SIP start date. For example, if the scheduled SIP start date is January 7, you can set up the SIP till midnight on January 6.
If you wish to change more SIP details you can refer to these FAQs:
Change SIP amount
Add/Remove funds (change any funds)