\documentclass[a4paper,12pt,oneside]{article}
\usepackage[style=apa,sorting=nyt,sortcites=true]{biblatex}
\usepackage{fancyhdr}
\usepackage{lastpage}
\begin{document}
\doublespacing
***TEXT AND REFERENCES OMITTED****
\newpage
\singlespacing
\printbibliography
\end{document}
There are two questions here. For the problem of changing the word
"References", see the FAQ:
http://www.tex.ac.uk/cgi-bin/texfaq2html?label=fixnam
On the other hand, it appears much larger than the rest of the text and
it is not centered because it is treated as a section title. Are you
sure that you want to change that?
Best regards,
Jose Carlos Santos
That's how the bibliography heading is supposed to be: an
unnumbered section.
Look at \defbibheading in biblatex's doc if you want to change it.
For example you can say
\defbibheading{talikarng}{\begin{center}References\end{center}}
in the preamble and then
\printbibliography[heading=talikarng]
Ciao
Enrico