How do I write above a left-right arrow?

\stackrel{\mathrm{DFT}}{\longleftrightarrow}

You can define an extensible "leftright arrow" by the method explained in Section 38 in Mathmode.pdf by Herber Voß. You can read it with texdoc mathmode on your machine or via this link.

\documentclass{article}
\usepackage{amsmath}

\makeatletter
\newcommand\xleftrightarrow[2][]{%
  \ext@arrow 9999{\longleftrightarrowfill@}{#1}{#2}}
\newcommand\longleftrightarrowfill@{%
  \arrowfill@\leftarrow\relbar\rightarrow}
\makeatother

\begin{document}

$F \xleftrightarrow{\text{DFT}} G$

$F \xleftrightarrow{\text{overlong text}} G$

\end{document}

The optional argument to \xleftrightarrow is for a "under label", so \xleftrightarrow[xyz]{} will have xyz under the arrow, while \xleftrightarrow[xyz]{abc} will have abc over and xyz under the arrow.

\documentclass{article}
\usepackage{mathtools} 
\usepackage{extarrows} 

\begin{document}
    $$\xleftrightarrow{DFT}$$ % mathtools
    $$\xlongleftrightarrow{DFT}$$ % extarrows
\end{document}