We know that our old (unrenewed) self was nailed to the cross with Him in order that [our] body [which is the instrument] of sin might be made ineffective and inactive for evil, that we might no longer be the slaves of sin.
Once you receive Christ as your Savior the old sinful nature within has been done away with as your spirit man is made new. There is, as the above scripture reveals, an old unrenewed nature that has to do with the soul (mind, will, and emotions), that James 1:21 tells us must be saved (delivered) from the old nature by receiving with meekness the implanted Word of God. It is this renewal of the mind that helps us to develop our spirit man, and in doing so this new man leads you in victory over the flesh.
Walk in the spirit, dominated by your spirit, and you will not fulfill the desires of the flesh. For the mind to be renewed you must receive the Word with meekness (complete humility). Simply put, you don’t question, argue with, nor doubt God’s Word. You simply believe what it says and act upon it. As James chapter one goes on to tell us, one who is a hearer of the Word but not a doer deceives themselves. But a doer of the Word will be blessed in what they do. One’s mind is not fully renewed until they begin to act upon the Word of God. As you do so you learn to walk in the spirit, allowing your spirit man to dominate, and you shall not fulfill the lusts of the flesh.