Well, if you have a ton of faces, you could use the decimate modifier to slightly reduce your number. Other than that, though, the only way I know of doing it is remaking the model.
I tried that with a cylinder mesh and it just made it all weird.
Decimate is only good for organic meshes. Un-Subdivide mode might work for some hard-surface cases. For round things: remodel them. You could also try Remesh.
By the way, what is Remesh?
Ignore Axo-Tak, he's talking about MilkShape.Make cylinders and press F6 to open a dialog to change the amount of vertices. 6 and 8 are most commonly used in models for Blockland.
16 is pretty common as well.
Yeah, if you are doing that ugly super detailed attempt