The first method uses positioning
and node distance=2cm and 4cm]
but I don't appreciate it.
/tikz/node distance=⟨shifting part⟩ (no default, initially 1cm and
1cm) The value of this key is used as ⟨shifting part⟩ is used if and
only if a ⟨of-part⟩ is present, but no ⟨shifting part⟩.
and
When the ⟨shifting part⟩ is of the form ⟨number or dimension⟩ and
⟨number or dimension⟩, it has (essentially) the effect of shifting the
node vertically upwards by the first ⟨number or dimension⟩ and to the
left by the second.
I think it's strange to shift first vertically and I prefer to make a shift manually by myself.
\documentclass{article}
\usepackage{tikz}
\usetikzlibrary{positioning}
\begin{document}
\begin{tikzpicture}
[align=center,node distance=2cm and 4cm]
\node[label=above:A] (A) {(1)};
\node[label=above:B1] (B1) [above right= of A] {($m+1$)};
\node[label=above:B2] (B2) [below right= of A] {($m+1$)};
\node[label=above:C] (C) [below right= of B1] {($2m-1$)};
\end{tikzpicture}
\end{document}
I prefer +(2, 2)
and ++(2, 2)
it's like above right
and it's more flexible. But It's fine to have a lot of possibilities !!!
\documentclass{article}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture} [align=center]
\path (0, 0) node[label=above:A] (A) {(1)}
+(2, 2) node[label=above:B1] (B1) {($m+1$)}
++(2,-2) node[label=above:B2] (B2) {($m+1$)}
++(2, 2) node[label=above:C] (C) {($2m-1$)};
\end{tikzpicture}
\begin{tikzpicture} [align=center,xscale=2] % or x=2cm
\path (0, 0) node[label=above:A] (A) {(1)}
+(2, 2) node[label=above:B1] (B1) {($m+1$)}
++(2,-2) node[label=above:B2] (B2) {($m+1$)}
++(2, 2) node[label=above:C] (C) {($2m-1$)};
\end{tikzpicture}
\end{document}

xshift=<length>
to indicate the amount of the shift.below(or above)
andright (or left)
dimensions separately likeabove right=0.7cm and 1cm of A
. You have to use\usetikzlibrary{positioning}
for this.