The first thing to do is understand that Pixels are not Dots. The best way to view it is that dots are physically printed, where pixels are virtual and what we massage in our respective image editors (PPI - Pixels Per Inch). Anyone who works with a lot of prints knows the difference but will be so used to knowing which form of "DPI" is actually meant that it's rarely noticed if you were to use the wrong one.
Without going too much into laborious detail, printing at this kind of size is likely to be CMYK or some variant where each dot will be one of those colours (Cyan, Yellow, Magenta, Key/Black.) You can change the 'dot' size in some processes but the actual colours remain the same and to get the illusion of continuous tone printers use a pattern of those dots at a much higher DPI level than the PPI of the image provided.
As part of the printing process it has to re-scale and convert the image from the DPI provided to the PPI fitting the printing mechanism (and sometimes other variables like the paper stock selected) in the same way it has to map the colour-space and sometimes colour model of the image received. Depending on the printer this might happen in hardware or in the driver, some printers have better tuned printer profiles than others and so the same model of printer may give you better results at one print-shop vs another.
From a technical perspective you don't need to do anything. You can upscale and use a tuned sharpening filter on your image but I've not seen evidence that they add enough to the image output to make it worthwhile (YMMV and the best thing to do may be to experiment.) Some print shops may apply something like this for you or it could also be implemented in the printer firmware.
The main thing is that you have to consider is actually 'viewing distance' - it's unlikely that you'd be viewing a 40x30" image from the same distance that you would a 20x15" for example, so you don't need such a high PPI value to get a decent output.