nmcli connection modify [id | uuid | path] <ID> <setting>.<property> [<value>]

missing value means setting the default value (deleting).

Examples:
nmcli con mod ethernet-2 connection.autoconnect no
nmcli con mod "Home Wi-Fi" wifi.mtu 1350
nmcli con mod "T-Mobile 1" gsm.apn internet
nmcli con mod 91782692-512e-4408-9572-667887319ef4 conn.perm user:cimrman

TODO:
 - allow modifying multiple properties (using single command)