You need to delete the line:
OleDbDataReader re = new OleDbDataReader();
because that class does not have a constructor. Which is exactly what the error is telling you. The reader is instantiate by calling ExecuteReader, http://msdn.microsoft.com/en-us/library/system.data.oledb.oledbdatareader.aspx
re = cmd.ExecuteReader();
OleDbDataReader re = cmd.ExecuteReader();
Your other error is caused by your SQL statement.
"UPDATE register SET passwd = @newPass WHERE user_id =@userid";
Were you are adding 3 parameters but only using 2 in your statement. Either add the other parameter to your statement or get rid of adding the other parameter.