As noted in the code below I can do my loop and read data from the cells but I don’t know how to modify a single cell.

function modifyDataInRange(){
  var range1 = SpreadsheetApp.getActiveSpreadsheet().getSheetByName('MyData').getRange("A2:J300");
  var checkData = range1.getValues();
  var j = 0;
  while(checkData[j][0]!=""){
    if(checkData[j][1] == "Old Data"){
      checkData[j][1] = "New Data";         //this does not write to the cell
      range1.setValue("New Data");          //this writes to every cell in the range
    }
    j += 1;
  }
}

Is there a way to do something like range1.setvalue([j][1], “New Data”)?