I am trying to understand how the following two statements are equivalent:
$$ \sum_{l=-\infty}^{\infty} h^*[l] \ R_{xx}[m+l] = \sum_{i=-\infty}^{\infty} h^*[i-m] \ R_{xx}[i] $$
I get that we made $m + l = i$, and plug this into the $h$ and $R_{xx}$. What I do not get is how the sigma on the right hand side is now all the sudden going from $i$, and not $i-m$. What happened to the $m$?
What is the rule on changing variables like this within summations?