# How to replace variables in an expression?

I have generated a matrix using Mathematica,

$\left( \begin{array}{ccc} \text{cos$\theta $} & -\text{sin$\theta $} & \text{cos$\theta $}\, \text{dx}+\text{dx}-\text{dy}\, \text{sin$\theta $} \\ \text{sin$\theta $} & \text{cos$\theta $} & \text{cos$\theta $} \,\text{dy}+\text{dy}+\text{dx}\, \text{sin$\theta $} \\ 0 & 0 & 1 \\ \end{array} \right)$

Now I want to change the variable names $dx$ to $xr$ and $dy$ to $yr$. How could I do this?

-