I am calling a subprocess multiple times parallely from main process using MNI f

Within th subprocess(which you want to cancel), configure a rule, which triggers when pv!value="Reject". And connect this to the End Event, that terminates the process.

Thanks Vivek for the response...But the value "reject" came from another subprocess instance and got updated in main process. All subprocess instances were invoked at the same time. So, how will the pending subprocess instance get this value "reject"?

When configuring the subprocess node in Main process, pass the pv holding "Reject" value(in main process) as byreference to subprocess. i.e., pv!InSubProcess=byreference(pv!InMainProcess)

Thanks a lot Vivek! The functionality is working fine as per your suggestion and the pending subprocess instances are getting terminated when status is updated to "Reject". However, subprocess node in the main flow (for which exception flow is configured to go back in case of "reject" by any subprocess instance) is going to the further tasks along with the rejection flow. Not able to figure out why this is happening...

Is there a specific purpose of configuring the exception flow? As you are now terminating the processes, within the subprocess itself. You don't actually need to configure the Exception flow.
After the Subprocess node, configure a gateway that checks for the rejection and then continue/end the main process as per the requirement.
If the sub process node is configured as synchronous, then the flow would move on when all the instances are completed.

Thanks Vivek! I used this approach and now my functionality is working completely...