Go into svrmgrml

svrmgrml>shutdown (or shutdown immediate)

svrmgrml>startup mount

That's it. (At this point it's not open.. therefore you are the only one with access...Not sure what would happen if another user went to try the same thing?..)

I'm assuming you know how to do the rest..
(change init.ora params.)
alter database archivelog;

or see:
http://hojohn.photo.net/ora81/DOC/server.815/a67772/archredo.htm#4718