How do you convert a 12-0-12 volt transformer to 16-0 volts transformer?

You can not convert the transformer without cutting the secondary windings. The secondary winding is a split secondary that is wound that way from the manufacturer. To accomplish what you asked would entail unwinding each side of the 12 volt winding half way. At this point solder a pigtail out to a terminal block. This would be your 6 volt tap. Then the coil would have to be rewound to its original state as before the unwind. This same procedure will then be done to the other side of the secondary. Done as a project, maybe, workable within ones capabilities, no.