i want to increase the sga

Dear all,Iwant to increase the SGA size
Memory size: 32768 Megabytes
    memory (driver not attached)
    virtual-memory (driver not attached)
    memory-controller, instance #0
    memory-controller, instance #1
    memory-controller, instance #2
    memory-controller, instance #3
This is my RAm on solaris 10
and my sga size is SQL> show parameter sga

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
lock_sga                             boolean     FALSE
pre_page_sga                         boolean     FALSE
sga_max_size                         big integer 4843346336
SQL>
so i want to increase the size around 20gb so how can i increase
i am also uploading the paremeter file
This is my oracle setting in etc/system file * End MDD root info (do not edit)