What the law could not do in that it was weak through the flesh, God did by sending His own Son in the likeness of sinful flesh, on account of sin: He condemned sin in the flesh,
that the righteous requirement of the law might be fulfilled in us who do not walk according to the flesh but according to the Spirit. 
For those who are according to the flesh set their minds on the things of the flesh, but those who are according to the Spirit, the things of the Spirit.

The Holy Spirit is also given to the believer; He dwells in each. If the believer then walks according to the law of the Spirit, that is in Christ, we are made free from the law of sin and death. The righteousness of the law can in this way be fulfilled in us. But there is a condition. We must walk not according to the flesh but according to the Spirit.
What is the walk according to the Spirit? It is not self-occupation, nor even occupation with the Holy Spirit. Walking according to the Spirit is occupation with the Lord Jesus Christ. If the believer ever looks to Christ, depends on Him, draws all he needs from Him, if Christ is His all - then the believer walks according to the Spirit. Then there is power over the old nature and the righteousness, demanded by the law is being fulfilled. And we must not overlook the fact that God’s love is mentioned in this blessed unfolding of our deliverance in Christ.
We also find a contrast between the flesh and the Spirit. While the believer is no longer in the eyes of God in the flesh, the flesh, however, is still in him as long as he has this mortal body. There is therefore a conflict between the Spirit and the flesh. Humanity falls into two classes, those who are according to the flesh, the unsaved; and those who are according to the Spirit, believers in Christ. A believer is called to walk according to the Spirit, in the sphere into which he is brought through grace. He may walk according to the flesh, but that does not put him back into his former state, when unsaved, he was in the flesh.

Arno C Gaebelein: The Annotated Bible