You can follow these simple steps:
Click on the SIP tab (center of the screen, besides "My Goals") → SIP Actions (besides the goal of your choice) → Custom SIP
Home page → Mutual Funds (either from the center of the screen or from the bottom tab) → Goal in which you have SIP running → SIP Center → SIP Actions → Custom SIP
You can also refer to these screenshots to guide you:
1. Click on the Mutual Funds section.
2. Click on the goal in which you wish to change the SIP Amount.
3. Click on "SIP Center".
4. Click on "SIP Actions".
5. Click on Custom SIP. To Add funds, click "Select Funds". To remove funds, click on the cross button beside the existing list of funds.
6. Review and confirm.
Please note that one goal can only have 1 SIP date, so any new funds that you add will also be invested in on the same SIP date.
If you wish to change more SIP details you can refer to these FAQs:
Change SIP date
Change SIP amount