Let's say we have an IP that is dropped by iptables 192.168.20.2
service iptables status|grep 192.168.20.2
184 DROP all -- 192.168.20.2 0.0.0.0/0
1.) Delete by the rule number which in our case is 184 from above.
iptables -D INPUT 184
2.) Delete based on the actual rule that we input to iptables
iptables -D INPUT -s 192.168.20.2/32 -j DROP
For example the rule would have been created using iptables -A INPUT -s 192.168.20.2/32 -j DROP so we just do the opposite to delete the rule.