How to add data to an array

I am learning c++ and i want to add data to an array from a existing array. I tried a lot but did not have any meaningfull codethat i can show here.

 int length = 1080;
 int width = 1920;
 char firstarray[ length * width * 4] = ( fillData );

this is a single dimension array i want to add data from this array to multidimensional array.

char [length][width * 4];