swap - swapping elements of a 2 dimensional vector c++ -

i have matrix of form

    vector<vector<int>> k 

which has size nxn. how can swap 2 elements of vector k[i]k[j] k[n-j][i]? in general, how can swap 2 elements of 2d vector?

because vector's [] operator returns reference, std::swap() work. example:

swap(k[i][j], k[n-i][i]);