How to restore Spfile from autobackup ?

Login to your database through RMAN prompt.

[oracle@ora ~]#  export ORACLE_SID=orcl

[oracle@ora ~]#  rman target=/

# Shutdown your database if it is open.

RMAN> shutdown immdiate

# Forcefully nomount your database.

RMAN> startup force nomount;

# give the destination of your controlfile autobackup,
# to restore your spfile 

RMAN> restore spfile from ‘/u01/app/oracle/flash_recovery_area
/ORCL/autobackup/2011_10_08/o1_mf_s_764006317_7909gpmx_.bkp’;

# Bounce or restart your database.

RMAN> shutdown immediate

RMAN> startup