SQL> ed
Wrote file afiedt.buf

  1  create or replace procedure test_user
  2  (user in varchar2,
  3   pass in varchar2)
  4  authid current_user
  5  is
  6  str varchar2(100);
  7  begin
  8  str:='alter user '||user||' identified by '||pass;
  9  execute immediate str;
 10* end;
SQL> /

Procedure created.

SQL> exec test_user('mag','mag');

PL/SQL procedure successfully completed.

SQL> connect mag/mag@rdtest;
Connected.
SQL> exec test_user('mag','newpass');

PL/SQL procedure successfully completed.

SQL> connect mag/newpass@rdtest;
Connected.