Heat the neck of the guitar with an iron on a low setting to melt the glue. You may wish to place a cloth over the guitar neck to prevent burning or ruining your iron.
Pull up on the neck of the guitar when the glue has melted. Wedge a flat wooden spatula between the neck and the guitar. Work your way up the neck until the old neck has been separated from the guitar.
Use a fine-grained sandpaper of about 250 grit to sand down and remove any remaining glue. Make sure no residue is left behind but avoid sanding down the actual wooden part of the guitar.
Apply aliphatic resin emulsion adhesive to the base of the neck and attach the new neck to the guitar. Make sure to align the new guitar neck to the guitar using the outline front the original neck.