Big theta notation in substitution proofs for recurrences
By definition, a function $f(n)$ is $\Theta(n^2)$ if there exist constant $b,c,N$ such that for all $n \geq N$,
$$ bn^2 \leq f(n) \leq cn^2.$$
In particular, as long as $n \geq N$, we can always replace $f(n)$ by $cn^2$ when upper-bounding an expression.
In your example of $cn^2 + n$, when $n \geq 1$ we have $n \leq n^2$ and so $cn^2 \leq (c+1)n^2$.