It depends what you mean by 'church.'
Reaching the unreached for the Kingdom of God is following the example of Christ and the apostles by obeying the commands of Christ and walking in Kingdom principles.
If you have to ask how the 'church' is to reach the unreached, then it is not the true body of Christ. The ekklesia Christ is building is already reaching the unreached where the true Gospel points to Christ and his leading.
My sheep hear my voice, and I know them, and they follow me. - John 10:27
If you want to reach the unreached with the genuine Gospel, start by seek the 'voice' of Christ and follow him an learn what it means to walk in Kingdom principles.