I tried two ways.
1. Alter system set memory_target = <> scope =spfile; and then restarted the whole cluster as it didn't allow me to reboot ASM instance as only one node.
2. created pfile from spfile and modified it. Then created a new spfile from pfile and deleted the old spfile. Then restarted the whole cluster.
Both ways nothing happens. When i start the cluster by default the memeory_target is 1GB. How this works??
One more point : when i create a pfile from spfile and i see the changed value of memory target but that is not in memeory even after restart of clsuterware. How is it possible. It seems +ASM instance is not reading the spile in ASM DISKS represented by spfile parameter.
I really need help.