Believers aren't in the flesh anymore, by virtue of having the Holy Spirit:
"you are not in the flesh but in the Spirit, if indeed the Spirit of God dwells in you." (Romans 8:9 NASB)
So the issue is not asking how believers overcome something that they aren't in anymore because the Holy Spirit is in them. They overcame that and became new creations when they were born again. The issue is asking how believers overcome the temptations of the bodies they still have.
But you are not in the Flesh (in one sense of the word is asking the question, "Since you are now a Believer and no longer depending on the flesh, why are you resorting to the flesh?"), But in the Spirit (as a Believer, you now have the Privilege of being led and empowered by the Holy Spirit; however, He will do such for us only on the premise of our Faith in the Finished Work of Christ), if so be that the Spirit of God dwell in you (if you are truly Saved). Now if any man have not the Spirit of Christ, he is none of His ( Paul is saying that the work of the Spirit in our lives is made possible by what Christ did at Calvary, and the Resurrection). (Romans 8:9)
Walking in the Spirit
16 This I say then, Walk in the Spirit, and ye shall not fulfil the lust of the flesh.
17 For the flesh lusteth against the Spirit, and the Spirit against the flesh: and these are contrary the one to the other: so that ye cannot do the things that ye would.
18 But if ye be led of the Spirit, ye are not under the law.
19 Now the works of the flesh are manifest, which are these; Adultery, fornication, uncleanness, lasciviousness,
20 Idolatry, witchcraft, hatred, variance, emulations, wrath, strife, seditions, heresies,
21 Envyings, murders, drunkenness, revellings, and such like: of the which I tell you before, as I have also told you in time past, that they which do such things shall not inherit the kingdom of God.
22 But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith,
23 Meekness, temperance: against such there is no law.
24 And they that are Christ's have crucified the flesh with the affections and lusts.
25 If we live in the Spirit, let us also walk in the Spirit.
26 Let us not be desirous of vain glory, provoking one another, envying one another.
Galatians 5:16-26
"Manifest" in the Greek is "phaneros," and means "open, evident." The idea is, that if the Believer resorts to Law, some part of these sins are going to be manifested in such a person, and despite their efforts otherwise. (Pastor Jimmy Swaggart Bible Commentary - Galatians)
If the Believer Faith is not maintained in Christ and the Cross, you will fulfill the lust of the flesh.