So if you have already done SIP through Fyers Direct, then you can not modified the existing SIP tenure as well as the amount invested. However if you want to increase the tenure of the same SIP and want to increase the SIP amount, then in that case you need to open a new SIP. This new SIP you can start without closing your previous existing SIP.
To make it more clear let me share an example with you.
Say suppose I have done a SIP in Axis Growth plan for 12 months with a initial amount of 5k, through Fyers Direct. Now say, I want to modify it and increase my time horizon from 12 months to say further 24 months, also I want to increase the amount to 10k in the same Axis Growth Plan.
So, through Fyers Direct I can not do this change . I need to start a new SIP in the same scheme. Also it is not compulsory to close your previous SIP. It will automatically get mature on time.
However for your knowledge if your doing SIP directly with an AMC , then in that case you can do the modifications in the existing SIP. With distributors such options is not available. However again depends from distributors to distributors. Some allows to do the modifications in the existing scheme.
Hope this solves your query.