Replace character at a particular position

[sonia.sardana]

function ConvertToUpperCase()
            {
            var result;
            var numaric = "sonia";
            var Character = numaric.charAt(0);
            result=Character.toUpperCase(); 
              //In Result char is coming in UpperCase
              //I want how to replace the charater at postion 0 in numaric with result
            }