Flip it!

Use nested for loops to traverse over the entire matrix. Remember this pattern for all 2D array problems :

int rows = matrix.length and int columns = matrix[0].length. Once you have the row / column dimensions, write 2 for loops that traverse the entire matrix. In the inner for loop, use a temporary variable to make the swap.

Here's the loop skeleton to help you out :

...

for(int i = 0; i <= r; i++){

 for(int j = 0; j <= c/2; j++){

   ... 

  (swap)

   ...

 }

}