Some photos I took with a 35mm SLR film camera were put onto a CD as digital prints at 90 dpi. I need to increase this to 200 dpi before I upload them to my store at Cafe Press. How can I increase the dpi? It's a shame that they didn't scan at a higher resolution. I do my own slides at 4,000 dpi to allow some easy touch-up work.
However... For your purposes, all you need to do is fit the Cafe Press's format. If you can control the image dimensions at all, just resize it to 200 dpi. It won't make the picture any BETTER, but it will at least display on the Cafe Press site correctly.
In Photoshop Elements, I just hit [CTRL]+[ALT]+[I] and then change the resolution and save. Image > Resize > Image Size | 