You may not be able to
Did you do everything in vector? Otherwise you can't. In FW or any other program. Once something is rastered ( or contains raster elements) you really cant up the resolution. So unless you planned ahead and made your document at a large enough resolution for print in advance (~200-300dpi), you are stuck.
IF you have your original FW PNGs ( not the optimized output) AND all your work was shape based/text ( no brushes or pics), you can go Modify>Canvas an input an appropriate DPI. Ask your print shop what their requirements are; they usually range from200dpi~300dpi. Also ask if they can work with RGB files. IF they say no, well you have run into another limitation of FW, but this one is easy to fix. Save the file a NATIVE FW PNG, then open it up in PS and change the color mode. Save again. Do note, that once yo do this IT WILL NO LONGER be a FW native file.
Hope that helps.