In order to use the cmd-set-dhcp-defaultLeaseTime-1h.xml file as is, following is how you should be using netconf-console:
$ netconf-console cmd-set-dhcp-defaultLeaseTime-1h.xml
To use the --edit-config argument of netconf-console, following is the man page info on the --edit-config argument:
--edit-config=EDIT Takes a filename (or '-' for standard input) as
argument. The contents of the file is data for a
single NETCONF edit-config operation (put into the
<config> XML element). Takes an optional --db
argument, default is 'running'.
If you follow the above man page information, you will modify the contents of the xml file that you supply with the --edit-config argument as follows:
<dhcp xmlns="http://tail-f.com/ns/example/dhcpd"
xmlns:nc="urn:ietf:params:xml:ns:netconf:base:1.0">
<defaultLeaseTime nc:operation="merge">
PT1H
</defaultLeaseTime>
</dhcp>