Can a 8 ohm speaker replace a 6 ohm speaker?

You need to find out how many ohm's your deck is pushing out and make sure you have the right grade speaker wire so you don't under power the speaker

Rubbish. A "deck" doesn't "push out" ohms.

An amplifier is designed to develop a stated power in a stated impedance, and that is based on the amplifier's ability to deliver current.

An amplifier designed to deliver maximum output to a 6 ohm load can safely drive an 8 ohm one. But it will not be able to develop its maximum power.