Q: I want to change icloud ID but the device is requesting for the previous owner's password to turn off 'find my iPhone" before I can log out and I can't reach the previous owner

Sorry but the only possible way is to contact the previous owner and ask him to remove this iphone from the list of his devices.

Find My iPhone Activation Lock: Removing a device from a previous owner’s account - Apple Support

Perceptiveness is the key and you got it.

If you have the ORIGINAL password to unlock it, then by all means use it.  Otherwise the device is useless to you or anyone else except the ORIGINAL owner.  Activation lock is an anti-theft feature.  If we knew how to bypass it, it wouldn't be much of a deterrent.  I'm not saying YOU are a thief.  This is all explained in this article:

Turn off Find My iPhone Activation Lock - Apple Support