To increase and decrease the amount, you just need to edit the current SIP.
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 --> Modify SIP --> Make the changes and confirm.
When you make changes to the SIP, the old SIP is stopped and a new one is setup with the desired changes. This won't affect your existing investments and the changes will be done only for your future SIP payments.
Please note that you would be able to setup an 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 setup the SIP till midnight on January 6.