in Evans while proving the mean value property for heat equation $(u_t-\Delta u=0)$ where $u\in C^2_1(U_T)$ they define $$\phi (r)=\frac{1}{r^n}\int\int_{E(r)}u(y,s)\frac{|y|^2}{s^2}dyds.$$ where $E(r)=E(0,0,r)$ and $E(x,t,r)=\{(y,s)\in R^{n+1}:\mu(x-y,t-s)\geq\frac{1}{r^n}\}$ is the heat ball and $\mu$ is the fundamental solution of the heat equation.
after that they applied change of variable to say that $$\phi (r)= \int\int_{E(1)}u(ry,r^2s)\frac{|y|^2}{s^2}dyds,$$ Can anyone please explain me how this change of variable take place. Any time of help would be appreciated. Thanks in advance