UPDATE MyTable
   SET MyColumn = REPLACE(MyColumn, 'MyOldSubstring', 'MyNewSubstring')
 WHERE Id = 10;

System.Data.OleDb.OleDbException was unhandled
  Message="Undefined function 'REPLACE' in expression."
  Source="Microsoft Office Access Database Engine"
  ErrorCode=-2147217900
  StackTrace:
       at System.Data.OleDb.OleDbCommand.ExecuteCommandTextErrorHandling(OleDbHResult hr)
       at System.Data.OleDb.OleDbCommand.ExecuteCommandTextForSingleResult(tagDBPARAMS dbParams, Object& executeResult)
       at System.Data.OleDb.OleDbCommand.ExecuteCommandText(Object& executeResult)
       at System.Data.OleDb.OleDbCommand.ExecuteCommand(CommandBehavior behavior, Object& executeResult)
       at System.Data.OleDb.OleDbCommand.ExecuteReaderInternal(CommandBehavior behavior, String method)
       at System.Data.OleDb.OleDbCommand.ExecuteNonQuery()
       ...

public void ExecuteNonQuery(string sql)
{
    OleDbCommand command = new OleDbCommand(sql);
    OleDbConnection connection = new OleDbConnection(ConnectionString);
    command.Connection = connection;

    try
    {
        connection.Open();
        command.ExecuteNonQuery();
    }
    catch
    {
        throw;
    }
    finally
    {
        connection.Close();
    }
}

UPDATE MyTable
   SET MyColumn = MID(
                      MyColumn,
                      1,
                      INSTR(MyColumn, 'MyOldSubstring')
                         - 1
                     )
                     + 'MyNewSubstring'
                     + MID(
                           MyColumn,
                           INSTR(MyColumn, 'MyOldSubstring')
                              + LEN('MyOldSubstring'),
                           LEN(MyColumn)
                              - INSTR(MyColumn, 'MyOldSubstring')
                              - LEN('MyOldSubstring')
                              + 1
                          )
 WHERE INSTR(MyColumn, 'MyOldSubstring') > 0
       AND Id = 10;