I'm using Fail2Ban on a server and I'm wondering how to unban an IP properly.

First, find the right jail to unban for. Use:

 fail2ban-client status 

to get the actual jail names. Then, use this to find the iptables rule name:

iptables -L -n

The rule name and jail name may not be the same but it should be clear which one is related to which.

fail2ban-client set <jail-name> unbanip <ip-address>