You can kill the previous mongod instance and start the new one.
To kill the previous mongod instance, first search for a list of tasks running on your machine by typing,
sudo lsof -iTCP -sTCP:LISTEN -n -P
Search for mongod COMMAND and its PID and type,
sudo kill <mongo_command_pid>
Now start your mongod instance by typing,
mongod
You can see MongoDB running successfully.