How to increase or decrease the ram(memory) of vm in XEN by xl config-update..?

How to increase or decrease the ram(memory) of vm in XEN by xl
config-update..?

i am experimenting xen 4.2.1 and its features.
xl config-update
i created the vm with memory of 512. and i entered into the vm, Now i want
to increse the memory to 1024 without destroying the VM. how to acheive
this.
In old config file:
memory = '512'
after update i want as:
memory ='1024'