How to return the original log retention after purge kafka topics

[#1]

we have hadoop cluster version 2.6.0.x with 3 kafka machines version - 0.10.0.2.6

we want to purge all data files from all kafka's topics ( purge completely all data )

the CLI for this action should be

kafka-topics.sh --zookeeper localhost:13003 --alter --topic Topic1 --config retention.ms=1000
kafka-topics.sh --zookeeper localhost:13003 --alter --topic Topic2 --config retention.ms=1000
kafka-topics.sh --zookeeper localhost:13003 --alter --topic Topic3 --config retention.ms=1000
kafka-topics.sh --zookeeper localhost:13003 --alter --topic Topic4 --config retention.ms=1000

since we Temporarily update the retention time on the topic to one second (1000 ms )

then how to return the previous original retention!

Note - log.retention.hours = 168 ( from the ambari GUI ) and this is the original value

so how to return the original value (168 hours) back on all topics ? after purge all Topics?