How to increase memory

[alikhani]

when I run the command "free"
the system says:
                   total             used          free
shared      buffers         cached
Mem:         511656        505620      6036         0
12440          382592
-/+ buffers/cache:          110588       401068
swap:                0          0                  0
now when users want to connect webmail on my server it is very slow and
since you see the memory usage is high and i need to increase the size
of memory .The VM Admini can increase the amount of memory to 1GB in VM
but how can i do this in linux up to 1GB or something else.
-regarding Sophia