How can we rename file name in data grid view adding prefix

int CheckBoxRowCounter = 0;
            for (int i = 0; i < dataGridView1.RowCount; i++) {
                if (Convert.ToBoolean(dataGridView1.Rows[i].Cells[0].Value)) {
                    CheckBoxRowCounter++;
                }
            }
            if (CheckBoxRowCounter > 0)
            {
                for (int i = 0; i < dataGridView1.RowCount; i++)
                {

                    if (Convert.ToBoolean(dataGridView1.Rows[i].Cells[0].Value))
                    {
                        string SourceFilePath = dataGridView1.Rows[i].Cells[1].Value.ToString();
                        string FileName = new System.IO.FileInfo(SourceFilePath).Name;
                        var prefix = Regex.Match(SourceFilePath, "^\\D+").Value;
                        var number = Regex.Replace(SourceFilePath, "^\\D+", "");
                        var r = int.Parse(number) + 1;
                        var newstring = prefix + r.ToString(new string('0', number.Length));
                    }
                }