First, I think you have to take a look at the word "evolve." What we have done to cotton has been of benefit to us, certainly, but I am not convinced that it counts as true evolution. Cotton was doing just fine before we came along.
The answer to your question is that humans have helped cotton to "evolve" in the same way they've helped most everything to:
- Scientists and farmers selectively breed crops in the same way that they do animals. An example is Tanguis cotton, which was created in Peru to survive a particularly nasty fungus. Agriculturalists take one plant and breed it with another related one with the hopes that the best qualities of both will mix.
- Scientists and farmers genetically alter the cotton to make it more profitable (again, I am not sure it counts as evolution, but in the sense it is more likely to survive perhaps it does.) An example of this would be cotton that has certain bacteria genes spliced in that make the plant unattractive to some insect pests.
In short, whether or not it counts as evolution, humans have changed cotton to make it yeild more, grow in more places, and be less susceptible to both diseases and insects.