How to change max server memory in SQL Server
Just start SQL Server with the -f switch, which will start it in a minimal, single-user mode. You should be able to change it back afterwards.
Use this link to start sql server in single user mode and set max server memory by using following command.
1.http://blog.sqlauthority.com/2009/02/10/sql-server-start-sql-server-instance-in-single-user-mode/
2.sp_configure 'Max Server Memory',1024 reconfigure
When you are starting sql server in single user mode you have to disconnect all connections.