Q:

Can I change one counter in an enumerate list?

I would like to do something like this:

First premise.
second premise.
Third premise.

C. Conclusion
(I can't even  do it in this comment box)

A:

As Werner said (two minutes ago) in the comment:

\documentclass{article}
\usepackage{enumitem}

\begin{document}

\begin{enumerate}
\item First premise.
\item second premise.
\item Third premise.
\item[C.] Conclusion
\end{enumerate}

\end{document}

A:

Two patterns for presenting arguments in standard form. The first is the one I tend to use, where the conclusion is also numbered. The second uses the 'C.' marker shown in your example. Both use the infer environment, defined in the preamble.
\documentclass{article}
\usepackage{enumitem,expdlist,setspace}
\newlist{inferenum}{enumerate}{10}
\setlist[inferenum]{label={\arabic*.},ref={\arabic*},partopsep=0pt,topsep=0pt}

\makeatletter
% %:infer (standard form)
\let\premise\item
\let\conc\item
\def\conclusion{\item[C.]}
\newlength{\trythis}
\setlength{\trythis}{\parindent}
\newcommand{\inferline}{}
\newenvironment{infer}%
  {%
    \pagebreak[3]%
    \begin{singlespacing}%
      \vspace{0pt\@plus\baselineskip}%
      \renewcommand{\listpartsep}{-.5\baselineskip}%
      \begin{inferenum}%
        \setlength{\itemsep}{.25\baselineskip}%
        \setlength{\parsep}{0pt}%
        \renewcommand{\inferline}{\listpart{\hspace{\trythis}\rule{25mm}{0.5pt}}}%
    }{%
      \end{inferenum}%
      \vspace{1\baselineskip\@plus\baselineskip}%
      \end{singlespacing}%
    }
\makeatother
\begin{document}

  \begin{infer}
     \premise First premise.
     \premise Second premise.
     \premise Third premise.
     \inferline
     \conc Conclusion.
  \end{infer}
  \begin{infer}
     \premise First premise.
     \premise Second premise.
     \premise Third premise.
     \conclusion Conclusion.
  \end{infer}

\end{document}

