I need to replace a text by another in a textBox can anyone help me...?

Well unless I have understood the question wrong, I would do text1.text= newText;.

I have the solution and work in IE,Firefox and chrome

this funtion remplace a selected string in textbox

example:

x=hello (the selected text in textbox)
v1=<strong>
v2=</strong>

<input type="button" value="Bold" onclick="remplace('<strong>','</strong>')" /> (input button)

function remplace(v1,v2)
   {
   
    var elementRef = document.getElementById("IDofTextBox");

     // IE ...
    if (document.selection) {
             str = document.selection.createRange().text
             document.selection.createRange().text = v1 + str + v2;
     }
     else if ((elementRef.selectionStart) || (elementRef.selectionStart == '0'))
     {
       // Mozilla/Netscape...

             var startPos = elementRef.selectionStart;
             var endPos = elementRef.selectionEnd;
             var str = elementRef.value.substring(startPos, endPos);
             elementRef.value = elementRef.value.substring(0, startPos) + v1 + str + v2 + elementRef.value.substring(endPos, elementRef.value.length);
     }
     else
      {
       elementRef.value += valueToInsert;
      }
   }

the result <strong>x</strong> inside of textBox

If you use master.page and add this code in a asp:Content you need this function to find the real Id of the control

function RealID(id)
   {
       var strId="";
       var elm="";
       for(i = 0; i < document.forms[0].elements.length; i++)
       {
           elm = document.forms[0].elements[i];
           if (elm.id.indexOf(id)!=-1)
           {
           strId=elm.id;
           return(strId);
           }
       }
   }

TextBox1.Text = "ABCABC"
string toReplace = "D";
string toFind = "B";
string Results = TextBox1.Text.Replace(toFind, toReplace);

//Results == "ADCADC";