How to change the numbering style for enumeration and sub-enumeration

Question

I wanted to use the enumerate environment. I am using this code:

\begin{enumerate}
    \item To define the outliers and events.
            \begin{enumerate}
                        \item How can events be defined?
            \end{enumerate}
    \item To detect an event.
    \item To characterize an event.
    \item To evaluate the detected outliers and events.
\end{enumerate}

result:

1. To define the outliers and events.

   (a) How can events be defined?

2. To detect an event.
3. To characterize an event.
4. To evaluate the detected outliers and events.

I wanted to replace the sub numbering of alphabet with numbers. Expected results:

1. To define the outliers and events.

   1.1 How can events be defined?

2. To detect an event.
3. To characterize an event.
4. To evaluate the detected outliers and events.

Answer 1

If you don't want to use a specialized package such as enumitem, you could proceed by modifying the \labelenumii ("label of level-2 enum items") macro, as is done in the following MWE (minimum working example).

\documentclass{article}
\renewcommand\labelenumii{\theenumi.\arabic{enumii}.}
\begin{document}
\begin{enumerate}
    \item To define the outliers and events.
            \begin{enumerate}
                        \item How can events be defined?
            \end{enumerate}
    \item To detect an event.
    \item To characterize an event.
    \item To evaluate the detected outliers and events.
\end{enumerate}
\end{document}

Answer 2

Easy job with enumitem package:

\documentclass{article}
\usepackage{enumitem}

\begin{document}
\begin{enumerate}
    \item To define the outliers and events.
            \begin{enumerate}[label={\arabic{enumi}.\arabic*}]        %% modified here
                        \item How can events be defined?
            \end{enumerate}
    \item To detect an event.
    \item To characterize an event.
    \item To evaluate the detected outliers and events.
\end{enumerate}
\end{document}

As suggested by Werner, for internal enumeration, you can also use label*={\arabic*}:

\documentclass{article}
\usepackage{enumitem}

\begin{document}
\begin{enumerate}
    \item To define the outliers and events.
            \begin{enumerate}[label*={\arabic*}]        %% modified here
                        \item How can events be defined?
            \end{enumerate}
    \item To detect an event.
    \item To characterize an event.
    \item To evaluate the detected outliers and events.
\end{enumerate}
\end{document}