pfile is the initialization parameters of DB in plain text, while spfile is the binary one, so to be able to change it, you have to:

1. Extract the pfile from the spfile:
   “create pfile=’my_path/file_name.ora‘ from spfile;”
2. Shutdown the DB:
   “shutdown immediate;”
3. Edit the pfile applying the new parameters with “vi”, “notepad” or any plain text editor
4. Startup the DB from the pfile:
   “startup pfile=’my_path/file_name.ora‘;”
5. Generate spfile from pfile:
   “create spfile from pfile=’my_path/file_name.ora‘”

Source: http://www.dba-oracle.com/concepts/pfile_spfile.htm