As a believer, you're destined to be molded into the image of Jesus, to grow in spirit until you look just like Him. God's supernatural power began that process the moment you were born again. You were changed on the inside instantly. How successful you are in allowing this change to progress to the outer man is mostly up to you. We grow up spiritually just like we grow up physically.
What can you do to speed this growing process along?
Spend more time pursuing the things of the Spirit than you spend pursuing the things of the flesh. Become dedicated to the Word of God and to fellowship with Him. Be willing to pull aside from the busy things of life and get alone with Him, so He can guide you, teach you and share His likeness with you.
Have you ever noticed that if you spend time around people who have strong personalities, you will automatically be affected by them? You'll find yourself doing the things they do. Their mannerisms will rub off on you. You can't help it. They just influence you, especially if they are people you respect and admire.
The same thing is true in your relationship with God. If you spend enough time with Him, He's going to rub off on you! His ways will become your ways.
So make yourself available to Him by praying in the spirit and worshiping and loving Him. Hang around His Word and the people who love His Word and live it.
Before you know it, you'll begin to notice yourself changing. You'll find your character becoming more like His. You'll begin to think like He does and talk like He does and even act like He does. His image will begin to shine forth in you!