I'm not sure you can change someone's personality. They have to want to adopt changes for themselves. The most you can do is provide an environment where they get to spend time with the opposite sex. (social gatherings, birthday parties, or enroll in classes where there is more student interaction such as drama class or music classes.) The more time one spends with the opposite sex the more comfortable they become around them.
However it is also important to prepare them for rejection. It happens to everyone but some people let it affect them for the rest of their lives. Best of luck!