You can't get detail that isn't there, you got to start over, sorry. Or you can just turn off resample image when you change the image size, but then the inches just go down in proportion to the increase in dpi. Dpi is meaningless in itself, when you work in Photoshop you work in pixels, and you always have to plan so you have enough for the end use. Plan ahead! If you have too many (always a good plan) you can resample down, at the end, but you can't resample up, all you are doing is making a bigger file.