Set the iron to medium heat. Place it on top of the existing fretboard to melt the glue.
Pull up on the bottom of the fingerboard, and insert a wedge underneath the fretboard to keep it propped up. Iron on top of the fretboard to continue melting the glue. Drive the spatula between the fretboard and the base, to loosen the fretboard from the guitar.
Purchase a new, pre-slotted fingerboard from a local music store or guitar specialty store. You may also purchase one from an online specialty store.
Sand down the old base to the fretboard, to remove any excess flue. Use a fine, 250-grit sandpaper.
Apply glue to the back of the fretboard, and use four clamps -- spaced evenly along the fretboard -- to hold it in place while gluing. Allow the glue to set for 24 hours before removing the clamps.