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
            }

function ConvertToUpperCase() {
var result;
var numaric = "sonia";
var Character = numaric.charAt(0);
result = numaric.replace( Character, Character.toUpperCase() ); // Converting target character to uppercase letter.
}

function ConvertToUpperCase()
            {
            var result;
            var numaric = "sonia";
            result=numaric.charAt(0).toUpperCase() + numaric.substring(1); 
            }

protected void Page_Load(object sender, EventArgs e)
    {
           try
           {
             if (!IsPostBack)
             {
           txtCompanyName.Attributes.Add("onblur", "javascript :  ConvertToUpperCase()");
 
 }
           }
           catch (Exception  ex)
           {
               lblStatus .Text =ex.Message .ToString ();
           }
    }
 
 
 function ConvertToUpperCase()
            {
            var numaric = document.getElementById ("txtCompanyName").value;
            numaric =numaric.charAt(0).toUpperCase() + numaric.substring(1);
            var pos = numaric.indexOf(" ");
            while(pos > -1)
                {
                  numaric =numaric.substring(0,pos+1) +  numaric.charAt(pos+1).toUpperCase() + numaric.substring(pos+2);
                  pos = numaric.indexOf(" ", pos+1);
                }
            document.getElementById ("txtCompanyName").value=numaric;
           }

//This is a general utility function that returns an Initial Caps version of the input string
function initialCaps(str){
	var a = str.split(' ');
	for(var i=0; i<a.length; i++){
		ch = a[i].charAt(0);
		a[i] = a[i].replace(ch, ch.toUpperCase());
	}
	return a.join(' ');
}
 
//This function is applies initialCaps() to a specified form field.
function field2InitialCaps(filedId){
	var f = document.getElementById (filedId);
	if(f) { f.value = initialCaps(f.value); }
}

<!-- Demo -->
<input id="txtCompanyName" type="text" value="sonia is a beautiful name" size="50" /><br />
<a href="" onclick="field2InitialCaps('txtCompanyName');return false">Convert to Initial Caps</a>