Margins with newlfm

I would personally advise against using newlfm specifically for this reason. Use a regular article document class and set your margins and style without hassle:

documentclass[11pt]{article}

usepackage{geometry,lipsum}
geometry{
  margin=1in
}

setlength{parindent}{0pt}% Remove paragraph indent

begin{document}

pagestyle{empty}% Remove page headers/footers

mbox{}hfill
begin{tabular}{l @{}}
  Address 
  of 
  Sender  
  today
end{tabular}

bigskip % Vertical skip between sender/receiver address

begin{tabular}{@{} l}
  Name and 
  Address 
  of 
  Receiver
end{tabular}

bigskip % Vertical skip between receive address and letter opening

Dear John,

medskip % Vertical skip between letter opening and letter body

lipsum[1]

medskip % Vertical skip between letter body and closing

hspace*{.5linewidth}%
Regards,

medskip % Vertical skip between letter closing and signature start

vspace{5baselineskip} % Content that includes your signature, or space for it

My signature

bigskip % Vertical skip between signature and attachment references

cc: % people this letter is cc-ed to

medskip

encl: % list of anything enclosed

medskip

ps: % any post scriptums

end{document}

The code and accompanying output reads better together as the content flow is sequential - you don't have to put the sender/recipient and signature detail in the preamble when they could easily fit within the document body.

With a plain article class, you can use geometry to set your margins as you need.

This command will modify the top margin.

topmarginskip{0in}
newlfmP{topmarginskip=-1in}

You can use negative values if you want. You can find other pertinent commands in the manual.

You can do it with setting newlfmP{headermarginsize=10pt}.