sql help needed

Hello everybody.
I have the following problem, hope somebody can help me. I have table in an Oracle database (Oracle7 Server Release 7.2.2.3.0) and I am accessing. modifying updating my table by using sqlplus (PL/SQL Release 2.2.2.3.0) on Solaris 2.5.
Ok, now the problem. I have to replace any character occurence on character fields. I cannot find an SQL commands to do that. UPDATE, as far as I have understood, does replace the whole field , but is there any way of replacing a character and not the field? Example: In the field I have the following string:

        gianmarco,paris
I'd like it to become:

        gianmarco_paris

I really haven't got a clue about a possible way of doing it. I would prefer not to export the whole table in ascii format, amend it and reload it (which can be done, but there must be another way) Thank you very much in advance
gianmarco

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Dr Gianmarco Paris                          
Dipartimento di Elettronica e Informazione   phone: +39-2-2399.3669 
Politecnico di Milano                        fax  : +39-2-2399.3412
Via Ponzio 34/5                              email: paris_at_elet.polimi.it
I-20133  MILANO


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