The steps to change the title of the table of contents (ToC) depends on if you are using the babel package or not.

Without babel

The title of the table of contents can be changed using the command \contentsname. Let’s see a small example:

\documentclass{article}
\renewcommand{\contentsname}{Index} % ToC will show "Index" instead of "Content"

\begin{document}

\tableofcontents

\section{Section}
\subsection{Subsection}

\end{document}

With babel

When using babel package the name of the table of contents needs to be changed for the particular language used with babel.

The following example shows how to change the ToC name for english language:

\documentclass{article}

\usepackage[english]{babel}

\addto\captionsenglish{
  \renewcommand{\contentsname}
    {Index} % ToC will show "Index" instead of "Content"
}

\begin{document}

\tableofcontents

\section{Section}
\subsection{Subsection}

\end{document}

So, we can replace the " english " in \captionsenglish with the language we are using in babel.