It is worth checking the replication factor before modifying:

./path/to/kafka/bin/kafka-topics.sh --describe --zookeeper localhost:2181 --topic test-topic

Topic:test-topic        PartitionCount:2        ReplicationFactor:1     Configs:
        Topic: demo-topic       Partition: 0    Leader: 0       Replicas: 0     Isr: 0
        Topic: demo-topic       Partition: 1    Leader: 1       Replicas: 1     Isr: 1

It can be seen here that the replication factor is 1.

To update to a replication factor of e.g. 3 firstly create a json file with a new reassignment plan. Call it e.g. reassignment-plan.json: