How can I delete Icloud account from previous owner without password

Does the iPhone have "Find Mi iPhone" enabled. Then the Activation Lock will prevent you from erasing the device and disabling this feature. You cannot do anything with the iPhone until the previous owner disabled the lock.

See: https://support.apple.com/en-us/HT201365

There is no way around this. It is a built-in security feature. Without the previous owner's cooperation the iPhone will be useless for you.

Ask the previous owner to follow this procedure: https://support.apple.com/en-us/HT201351

If that is not possible, return the iPhone to the person who sold it to you.