from Cameron Roberts answer that works in almost every cases (if all cells are filled with strings only) and just for typing convenience, here is the same script with a small change in the map function : I added toString() to the return argument.

function testReplaceInSheet(){

    var sheet = SpreadsheetApp.getActiveSheet()

    replaceInSheet(sheet,'values','test');

}



function replaceInSheet(sheet, to_replace, replace_with) {

  //get the current data range values as an array

  var values = sheet.getDataRange().getValues();



  //loop over the rows in the array

  for(var row in values){



    //use Array.map to execute a replace call on each of the cells in the row.

    var replaced_values = values[row].map(function(original_value){

      return original_value.toString().replace(to_replace,replace_with);

    });



    //replace the original row values with the replaced values

    values[row] = replaced_values;

  }



  //write the updated values to the sheet

  sheet.getDataRange().setValues(values);

}