I have now participated in, or led, a renewal of baptism service five times in 2016. It's a great reminder of God's abundant grace at work in our lives, and our response to it. The most commonly used service is in the United Methodist Hymnal, beginning on page 50. Here's an updated version of the renewal service, which I found on umcdiscipleship.org:
While baptism in the United Methodist Church is a sacrament, meaning only ordained elders can offer it, the renewal of baptism service can be led by anyone. Water is offered, and the person dips their hands in it, maybe forms a cross on their forehead, or playfully splashes the water. What is important is that in the United Methodist Church we do not practice a "second baptism" or a "re-baptism." We believe God acts in our baptisms, regardless of our age or capacity to express faith. So the leader in offering this service should make it clear that the sacrament of baptism is not being offered. Those who have not been baptized but are interested should speak with their pastor.