You'd need to know the exact current involved, or use a pot, etc. But, that would turn into straight heat. Not the best way to do it. Based on the power, you're looking at about 3A of current max? That'd come out to about 6W of power, that's a very large resistor, and you'd need some headroom over that. Heat sink, etc. I wouldn't go that route.
The 'right' way would be to actually change the output. How to do that all depends on how the thing is designed. It could be as simple as changing the regulation circuit, or you may need to change other components, i.e. transformer, input cap, resistors here and there, etc. Most likely could not be done without a schematic and some good knowledge of power supplies.
The most efficient way would probably be to buy the right adapter.
Though, other people here may have a clever idea that I'm not thinking about, don't give up hope, hehe.
