about database copy??

hi all
i want to copy the all database to another directory but i wnat to change the sid.
it means that the two database are the same besides the sid here r my steps

1.backup controlfile to trace
2.shutdown database
3.copy all datafiles,redo log files,controlfiles
4.modify the control file
5.svrmgrl
6.connect internal
7.run the control file

any one can tell me 4.how to modify the control file any one can help me
thank u very much

                                          kk