In Simple language,
ECU Remap is like loading a cracked version of Windows Ultimate when you have only limited resources.
You have a stock car , so everything on the car is limited, even the Revs are limited that you cannot use your car's potential. In that case you can RE-map Ecu , or install a Standalone Engine management system.
Depending on the cars (Sports mostly) can get a standalone plug and play unit.
ECU can be remapped yourself (don't know about your car but for my car in needs Consult cable) , engine management systems have their own softwares to tweak .
Standalone engine managment systems (Haltech, Apexi Power Fc, Bee-R etc etc ) are expensive.. your looking for about 2000 dollars ( 1 lac rupees)
It really narrows it down to what you want and how much your willing to spend.