User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:0.9.3) Gecko/20010816
Did you recompile ns after to have changed ns-default?
Anyway you should be able to set it from OTcl level
set RenoAgent [new Agent/TCP/Reno]
$RenoAgent set tcpTick_ 0.001
If it doesn't work the last chance is to check it in tcp.h/cc.
Bye
Sergio