How to change LTM sshd default service port

Remember that there are two kinds of interfaces you can connect by SSH on. One is the management interface (eth0/mgmt) and second is the tmm interfaces (the self IPs on the configured VLANs essentially) if permitted in the port lockdown settings.

Try using the modify command to alter the port statement in tmsh:

(tmos) modify sys sshd port 23
(tmos) list sys sshd all-properties
sys sshd {
    allow { ALL }
    banner disabled
    banner-text none
    description none
    inactivity-timeout 0
    include none
    log-level info
    login enabled
    port 23
}

You won't be kicked out of your existing SSH sessions, but the next connections will need to use the new port.