You can follow these simple steps:
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 → Modify SIP
Click on the SIP tab (center of the screen, beside My goals) → SIP Actions (besides the goal of choice) → Modify SIP
You can also refer to these screenshots to guide you:
Step 1. Click on the Mutual Funds section.
Step 2. Click on the goal in which you wish to change the SIP Amount.
Step 3. Click on "SIP Center".
Step 4. Click on "SIP Actions".
Step 5. Click on "Modify SIP". Here, you can change the amount for any fund, and confirm the new SIP amount. You can also remove or add more funds (select funds).
Please note that you can make any changes in the SIP up to 1 day prior to the scheduled SIP start date. For example, if the scheduled SIP start date is January 7, you can change the amount or make any other changes till midnight of 6 Jan.
If you wish to change more SIP details you can refer to these FAQs:
Add/Remove funds (change any funds)