Q: My Iphone 5 has locked to icloud of previous owner after installing ios 9.3 and the previous owner refuses to unlock the phone

There is no way to downgrade and there is no way to bypass the activation lock as well. Contact the previous owner and try persuade him to remove this iPhone from his list of devices. Otherwise, your iPhone will be a useless brick.

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

This is user to use forum. Apple representatives are rare visitors here. Nothing strange in this situation. It is the matter of your relations with the previous owner of this device. If you have all documents confirming the ownership take it to the Apple Store.