Prove that H(s) = G(!s) is a CPRNG if G(s) is one
A proof by contradiction seems most appropriate here.

Assume that $H$ is not cryptographically secure. This means it fails to provide at least one of the properties of a CSPRNG as defined in your class, textbook, whatever. Then you must show how you can use an attack on $H$ to attack the cryptographic security of $G$. To do this, note that $H(!s)=G(!!s)=G(s)$. Since $G$ is cryptographically secure, your assumption must be incorrect. Therefore, you can conclude that $H$ is in fact cryptographically secure.