It would be a lot easier to download
the trial version of TOAD
scratch that previous suggestion, I just tried out the Oracle SQL Developer link suggested and it works peachy fine for editing procs.
for SQLPlus you pretty much need to display the contents of the stored proc, and spool it to a file (as suggested by DCookie :-)) :
sqlplus> spool myprocname.sql;
sqlplus> select text from all_source where name = 'MYPROCNAME' and type = 'PROCEDURE' order by line;
sqlplus> quit;
then edit the local SQL file in a decent editor.
Then use SQLPlus to run the SQL file to re-build the proc for testing.
>sqlplus username/password@tnsnamesentry @myproc.sql
In short, a massive pain in the keester. :-)
Ron