Yes you can Modify the target and SL by clicking on modify button on the order book (F3). Check the Link it has few example's and in detail about how the Bracket Order & Trailing Stoploss works.
This has been explained in Bracket Orders and detailed page in Zconnect as Anil has pointed out. Please read it in case you need more clarification.
Also, the conversations over there say that only SL and Target can be modified, not the Trailing Stop Value. So if you have set your initail Trial SL value to 1 absolute point, you cannot later change it to 0.5 points, I guess.