I don't have contact with previous owner of iphone 5s? how can i deactivate icloud on it?

You cannot.

Only the prior owner can do this.

Sorry.

Until/unless the prior owner removes it from their account, it is useless to you.

You cannot. Only the owner that has the lock on the device can remove it. Apple will not assist either.

You can't. If you have the receipt from when it was originally purchased from Apple or an authorized reseller you can contact Apple and have them help you. If you do not there is nothing you can do.