How to decrease the font size of 'References'?

You wrote,

How to decrease the font size of 'References'? I want to make it 11pt fontsize.

I guess you want 11pt because that's the main document font size, right? If so, you may achieve your formatting goal by loading the etoolbox package and running the instruction

patchcmd{thebibliography}{section*}{subsubsection*}{}{}

in the preamble.

By default, the header of the bibliography section in the article document class employs the instruction section*, which creates an unnumbered section-level header which, in turn, employs the Large relative font size. The command shown above tells LaTeX to use the subsubsection* command instead, which will employ the normalsize relative font size which, as you may have guessed by now, works out to be 11pt for the document at hand.

documentclass[a4paper,oneside,11pt]{article}
usepackage[hmargin=2.5cm,top=4cm,bottom=2.7cm]{geometry}
usepackage{amsmath,amssymb}
%%usepackage{amsfonts} 'amsfonts' is loaded by 'amssymb'
usepackage{enumitem}
usepackage{array}
%%usepackage{mathptmx} 
usepackage{newtxtext,newtxmath}
usepackage{changepage}
usepackage{hyperref}

%% new:
usepackage{etoolbox}
patchcmd{thebibliography}{section*}{subsubsection*}{}{}

begin{document}

section*{References}        % use "Large"
subsection*{References}     % use "large"
subsubsection*{References}  % use "normalsize"
textbf{References} % <-- use "normalize" by assumption
bigskiphrule

begin{thebibliography}{9}
    
bibitem{yyy} A. Author, emph{Book Title}. Publisher, City, 2nd ed., 2020.
    
end{thebibliography}
end{document}