Hello Rajib,
You should have two outcomes: Approve and Reject.
From there on you should have two buttons on your task screen where the user can chose those two outcomes.
On your workflow you should have something like this:
Two outcomes where one goes forward and the reject will do a jump activity back to the previous step.
I censored the names of the flows and steps for obvious reasons, but this should give you some idea, good luck!
Thijs