What is the complexity to break second preimage resistance?
The right terminology is second preimage resistance and preimage resistance. 

Edited to reflect comment by otus. Preimage attack takes $O(2^{n})$ hash function calls on average.

Second preimage attack takes just one extra call so the complexity is essentially the same as that of the preimage attack. Given $x$ you want $x'\neq x$ such that $H(x)=H(x').$ So evaluate $H(x)$ for one extra query, and then apply the preimage attack to $y=H(x).$