To quote from Evans' PDE book pg 53,
Let $u \in C_1^2(U_T)$ solve the heat equation. Then \begin{equation} u(x, t) = \frac{1}{4 r^n} \int \int_{E(x, t; r)} u(y, s) \frac{|x - y|^2}{(t - s)^2} dy ds. \end{equation}
In the proof of this theorem, the book says, "Upon mollifying if necessary, we may assume $u$ is smooth." I'm not sure how this is done (do we just replace $u$ everywhere with $u^\epsilon$? But then does the theorem statement have to be revised to have $u^\epsilon$ instead of $u$?) or why this is necessary (we already are given that $u \in C_1^2 (U_T)$).