God does this work through the Holy Spirit. We can collaborate with God in this process by our full obedience to Him.
God wants to change you. If you are shy today, God wants to see you bold. If you are doubtful today, God wants to see you full of faith. Even if you are shy and pretend being as such because you inherited this quality from your ancestors and relatives – let me tell you, God desires to change it in you. He will accomplish this task if you allow it to Him.