Fields value doesn't update when I change parsing rule in logstash

Hi fellas , I am newbie here . When i change field value in logstash config file and restart logstash some time I can see the changed value but some time it doesn't work. Just for an example i simply added a static field in logstash config like this

add_field => ['vendorname','kibana']
and next time when I update this to add_field => ['vendorname','abc'] and restart logstash then It is not necessary I will see an updated value. And want to know how can i clear session of kibana

Such a configuration change only applies to new events. When you experience that the old value is still used, are you sure you're looking at events processed after Logstash was reconfigured?

what events are ? I just have conditional statement in config file

I'm talking about the events that Logstash processes and that you're viewing in Kibana. I don't find it likely that Logstash keeps adding the old field value after you've restarted it with a new configuration.

I am facing this problem and thought It may be cause of cache .

No, that's not likely.