In one slide, I find that two paragraphs are too close vertically. Then I want extra space. However, I find that applying more \ \ does not work in beamer.
Not a trivial question: see this TeX.SE post and this other.
The way to set the default parskip may be the following:
\documentclass{beamer}
%\usetheme{CambridgeUS}
\usepackage[english]{babel}
\parskip=20pt
\begin{document}
\begin{frame}
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed
do eiusmod tempor incididunt ut labore et dolore magna aliqua.
Ut enim ad minim veniam, quis nostrud exercitation ullamco
laboris nisi ut aliquip ex ea commodo consequat.
\end{frame}
\end{document}
Otherwise, if you want to change the parskip only locally (as I seem to understand), you can use the above syntax inside your frame:
\documentclass{beamer}
%\usetheme{CambridgeUS}
\usepackage[english]{babel}
\parskip=20pt
\begin{document}
\begin{frame}
Lorem ipsum dolor sit amet, consectetur adipiscing elit, sed
do eiusmod tempor incididunt ut labore et dolore magna aliqua.
\parskip=20pt
Ut enim ad minim veniam, quis nostrud exercitation ullamco
laboris nisi ut aliquip ex ea commodo consequat.
\parskip=0pt
Ut enim ad minim veniam, quis nostrud exercitation ullamco
laboris nisi ut aliquip ex ea commodo consequat.
Ut enim ad minim veniam, quis nostrud exercitation ullamco
laboris nisi ut aliquip ex ea commodo consequat.
\end{frame}
\end{document}
The second \parskip=0pt sets all the following occurrencies of this length back to 0 pt (as I think 0 is the default here otherwise).