ImageMagick is your friend:
pljvalez is correct on both counts:
a) You can't "increase resolution" beyond what the picture was originally taken yet (let's not talk about image processing techniques - that's really not relevant to this discussion)
b) "dpi" (and, for that matter, "ppi") can be confusing. In this context, they really don't necessarily "mean" anything!
Here's a link that might help clarify:
But if you want to make the image "smaller" ... or "larger" ... or you want to "sharpen" it - or any of a million other things - then ImageMagick is definitely your tool!