\documentclass{article}
\usepackage{amsmath,amssymb}
\usepackage{accents}
\usepackage{yhmath}
\begin{document}
First code using \verb|accents| package:
\[\accentset{\sim}{W}^{j}\]
Second code using \verb|yhmath| package:
\[\widetilde{W}^{j}\]
\end{document}