To change it for all nodes:
\tikzset{every node/.style={<style_specs>}}
If you have a custom style, you can set this only for that specific node style:
\tikzset{My Style/.style={red, draw=blue, fill=yellow!20, minimum size=0.5cm}}
If you want to change an existing node style, you can use .append style to add to the nodes style specs:
\tikzset{rectangle/.append style={draw=brown, ultra thick, fill=red!30}}

Code:
\documentclass{article}
\usepackage{tikz}
\tikzset{My Style/.style={red, draw=blue, fill=yellow!20, minimum size=0.5cm}}
\tikzset{rectangle/.append style={draw=brown, ultra thick, fill=red!30}}
\begin{document}
\begin{tikzpicture}
\node [My Style] at (0,0) {$x$};
\node [My Style] at (1,0) {$y$};
\node [rectangle] at (2,0) {$z$};
\end{tikzpicture}
\end{document}
This actual intent here is more complex that what I thought since a label is actually a node. One solution would be to define a style for each particluar shape. So, for instance, for a rectangle you could define My Rectangle as:
\tikzset{My Rectangle/.style={
rectangle, draw=brown, fill=yellow, thick,
prefix after command= {\pgfextra{\tikzset{every label/.style={blue}}}}
}
}
which alters the label style (Again, thanks to percusse):

You could also just change the rectangle style as follows:
\tikzset{rectangle/.append style={
prefix after command= {\pgfextra{\tikzset{every label/.style={blue}}}}
}
}
Code:
\documentclass{article}
\usepackage{tikz}
\tikzset{My Rectangle/.style={
rectangle, draw=brown, fill=yellow, thick,
prefix after command= {\pgfextra{\tikzset{every label/.style={blue}}}}
}
}
\begin{document}
\begin{tikzpicture}
\node [rectangle, label=black text] at (1,0) {$z$};
\node [My Rectangle, label={blue text} ] at (3,0) {$z$};
\node [rectangle, label={blue text} ] at (5,0) {$z$};
\end{tikzpicture}
\end{document}
\tikzsetdefines a new style. If you want to have it applied ALL the time, then you can use\tikzset{every node/.style={My Rectangle}}. If that doesn't solve your question, perhaps you need to ask a new question. – Peter Grill May 17 '21 at 17:26