Yes they can by helping those who are working for a democratic government. And when Mugabe and the others cry racism, laugh in their lying, evil faces and don't run like a bunch of rabbits.
You're suffering under the common illusion that democracy equals good government. Most of the founding fathers would have disagreed with you. Mugabe is an elected leader, and although he certainly fixed the last couple of elections, he has a great deal of popular support. Even if he were clearly the choice of the majority of people, would that make him a better leader? Of course not.
Why anyone thinks that a mob of largely ignorant, unsophisticated, heavily propagandized, superstitious and primitive people can magically find and elect a set of enlightened leaders, when far more advanced societies have managed to come up with the likes of Hitler, is beyond me.
Democracy is a good system when it leads to individual liberty, but democracy neither guarantees individual liberty, nor is necessarily the most likely system to produce it. Africa would be far better off under a set of benign colonial dictatorships, but I can't see the remotst chance the world will provide them with such.