As you probably know...
improve: make or become better
enhance: intensify, increase, or further improve the quality, value, or extent of.
Usually, enhance is used when a certain quality is being improved or exagerated. ex.
This makeup will enhance your features/ Enhance the effectiveness of your site.
While improve is used when you're making something better. Because of their similar definitions sometimes you can use then interchangeably. ex.
I'm going to improve my worth ethic/ The prime directive needs improving.