Before and after:
\documentclass{beamer}
\beamertemplatenavigationsymbolsempty
\usepackage{verbatim}
\usepackage{tikz,stackengine}
\setstackEOL{\\}
\usepackage{pgfplots}
\usetikzlibrary{shapes.geometric}
\begin{document}
\begin{frame}[t]
\frametitle{}
\begin{tikzpicture}[scale=.8, transform shape]
\node[draw, trapezium, red, rotate=-90, trapezium stretches body,
text width=3cm, align=center] at (0,0) {\rotatebox{90}{Investment}};
\end{tikzpicture}
\begin{tikzpicture}[scale=.8, transform shape]
\node[draw, trapezium, red, rotate=-90, trapezium stretches body,
text width=3cm, align=center] at (0,0) {\rotatebox{90}{%
\color{red}\Centerstack{My\\Big\\Investment}}};
\end{tikzpicture}
\end{frame}
\end{document}

Here is a version without addition packages, using a \parbox. However, the result is extremely sensitive to the \parbox width, here chosen precisely as 1.75cm:
\documentclass{beamer}
\beamertemplatenavigationsymbolsempty
\usepackage{verbatim}
\usepackage{tikz}
\usepackage{pgfplots}
\usetikzlibrary{shapes.geometric}
\begin{document}
\begin{frame}[t]
\frametitle{}
\begin{tikzpicture}[scale=.8, transform shape]
\node[draw, trapezium, red, rotate=-90, trapezium stretches body,
text width=3cm, align=center] at (0,0) {\rotatebox{90}{Investment}};
\end{tikzpicture}
\begin{tikzpicture}[scale=.8, transform shape]
\node[draw, trapezium, red, rotate=-90, trapezium stretches body,
text width=3cm, align=center] at (0,0) {\rotatebox{90}{%
\parbox{1.75cm}{\centering My\\Big\\Investment}}};
\end{tikzpicture}
\end{frame}
\end{document}

stackengineis standard for most distributions. – Steven B. Segletes Apr 11 '19 at 10:59\parbox. Unfortunately, the specified width, if not chosen just so, will change the trapezoid shape. – Steven B. Segletes Apr 11 '19 at 11:07\centeringat the appropriate spot. – Steven B. Segletes Apr 11 '19 at 11:20