You won't be able to immediately alter your SIP/STP or SWP amount. You can begin a new SIP/STP or SWP for the additional amount if you want to raise your SIP/STP or SWP amount. Therefore, you must start a new SIP for Rs. 500 and continue existing SIPs if you already have a SIP running for Rs. 1000 and want to increase your investment to Rs. 1,500.
You must terminate your current SIP and begin a new SIP with the lower amount if you wish to lower your SIP/STP or SWP amount. Go to Portfolio -> Active SIP to cancel a SIP/STP or SWP. A list of all currently running SIP/STP or SWP will be available. Your future transactions won't be executed if you select the order that you want to cancel.
Please be aware that if you terminate a SIP, your current investment won't be liquidated. The sum that has been invested in the fund will stay there. But the investment from the following instalment will end.