Eph. 2:

2 Wherein in time past ye walked according to the course of this world, according to the prince of the power of the air, the spirit that now worketh in the children of disobedience

This refers to the evil spirit and works on unbelievers.

What is it?

It is not connected to the Holy Spirit. On the contrary, it obeys the devil in Satan's kingdom.

Ephesians 6:

12 For our struggle is not against flesh and blood, but against the rulers, against the authorities, against the powers of this dark world and against the spiritual forces of evil in the heavenly realms.

How does it work?

It is well-organized, headed by Satan. He could send evil spirits to possess people and make them act physically, Mark 5:

1They went across the lake to the region of the Gerasenes. 2When Jesus got out of the boat, a man with an impure spirit came from the tombs to meet him. 3This man lived in the tombs, and no one could bind him anymore, not even with a chain. 4For he had often been chained hand and foot, but he tore the chains apart and broke the irons on his feet. No one was strong enough to subdue him. 5Night and day among the tombs and in the hills he would cry out and cut himself with stones.

They could also make suggestive thoughts in people's minds, 2 Corinthians 11:

3 But I am afraid that just as Eve was deceived by the serpent’s cunning, your minds may somehow be led astray from your sincere and pure devotion to Christ.

That's the bad news but there is good news, Ephesians 4:

20 That, however, is not the way of life you learned 21when you heard about Christ and were taught in him in accordance with the truth that is in Jesus.

How could the mind be 'renewed'?

22 You were taught, with regard to your former way of life, to put off your old self, which is being corrupted by its deceitful desires; 23 to be made new in the attitude of your minds; 24and to put on the new self, created to be like God in true righteousness and holiness.

Christians have been born again with the Holy Spirit living in us. The Holy Spirit is connected to our human spirit which in turn is in contact with our mind. That's the hardware equipment.

What does it mean practically?

Paul continues:

25 Therefore each of you must put off falsehood and speak truthfully to your neighbor, for we are all members of one body. 26“In your anger do not sin”: Do not let the sun go down while you are still angry, 27and do not give the devil a foothold. 28Anyone who has been stealing must steal no longer, but must work, doing something useful with their own hands, that they may have something to share with those in need.

29 Do not let any unwholesome talk come out of your mouths, but only what is helpful for building others up according to their needs, that it may benefit those who listen. 30And do not grieve the Holy Spirit of God, with whom you were sealed for the day of redemption. 31Get rid of all bitterness, rage and anger, brawling and slander, along with every form of malice. 32Be kind and compassionate to one another, forgiving each other, just as in Christ God forgave you.

The mind is the battlefield. Both the evil spirits and the Holy Spirit work on people's minds. Evil spirits work from outside to inside. Holy Spirit works from inside to outside. Evil spirits disturb people's minds. Holy Spirit gives us peace in our minds.