You can Add/Remove any funds from your SIP or even increase/decrease the amount in any funds.
On the Website: Click on the SIP tab (center of the dashboard) > Click on "SIP Actions" beside the goal where you want to make changes > Click on "Custom SIP".
On the App: Click on the goal where you want to make changes > Click "SIP Center" > Click "SIP Actions" > Click on "Custom SIP".
The Custom SIP page will show you the list of funds in your existing SIP and from here, you can choose to add any funds, remove any funds, change the amount for any of the funds, etc. Once you've made the respective changes, you can review and save these.
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