Can I upgrade the speaker in my iPhone?

Yes, it can be done. It would take quite a load of work to first find the correct speakers with similar dimensions and then dismantle the iphone and replace it and making sure that it works! You're better off just using external speakers or a dock! The quality difference would be too small to actually bother.

It'd be very difficult to replace the speaker assembly in an iPhone, but that's not to say it can't be done if the right replacement parts were available: the latter is a question for the awesome iFixit team =).

The internal speaker is based on a plastic chamber located at the back of it improving the sound quality. If you change only the speaker,without any change of the chamber, the result may be no mentional.Redesigning the chamber is out of a common customer's ability and will, as well as the result doesn't worth the effort.