how do we open sim lock is it cost to open sim lock ?

Only the carrier it's locked to can remove the carrier lock.

Contact them. Some carriers charge, some do not, some will not unlock at all.

i need help to unlock my iphone 6S Sim side

So what did u do??

Both of you should read Kilted Tim’s advice.

I have to unlock my network I’m not able to use my iPhone 6s Plus

As has been stated numerous times, Apple does not unlock phones. You must contact your cell service provider for help with that. The SIM card is locked by your provider, not Apple.