\documentclass{article}
\usepackage{amsmath}
\begin{document}
\[ \underrightarrow{\mathrm{word}} \rightarrow  \underleftarrow{\mathrm{word}} \]

\[ \overleftrightarrow{\mathrm{word}} \leftarrow  \underleftrightarrow{\mathrm{word}}  \]

\[ a^2 \xrightarrow{\mathrm{over}} b  \xleftarrow{\mathrm{over}} c \]

\[ a \xrightarrow[\mathrm{under}]{\mathrm{over}} b  \xleftarrow[\mathrm{under}]{\mathrm{over}} c \]

\[ a \overset{\mathrm{over}}{\rightarrow} b \underset{\mathrm{under}}{\leftarrow} c \]
\end{document}