I have wondered how to undefine existing commands in $\LaTeX$ for so long. Finally, I googled it up. It's easy. Simply

\makeatletter
\let\command\@undefined
\makeatother

and the \command has been undefined. This does not cause an error when \command was undefined before. After that, you can merrily

\newcommand{\command}{Hell Yeah.}

and be on your way.