I know, you have XenCenter and everything is easily done from there.
But what happens if you are not able to use it? Time to fire up the command line :)
In the example below, we will change the memory from 256M to 1G.

Commands to be executed:

xe vm-shutdown name-label=<vm>
xe vm-list params=uuid name-label=<vm>
xe vm-list params=all name-label=<vm> | grep memory
new_mem=1073741824; xe vm-param-set memory-static-max=${new_mem} memory-dynamic-max=${new_mem} memory-dynamic-min=${new_mem} uuid=<vm uuid>
xe vm-list params=all name-label=<vm> | grep memory
xe vm-start vm=<vm>
xe vm-list params=all name-label=<vm> | grep memory

Detailed execution:

Shutdown the vm and list the vm's current memory value and uuid:

[root@XenGz cd]# xe vm-shutdown name-label=debian
[root@XenGz cd]# xe vm-list params=all name-label=debian | grep memory- | grep -v recommendations
                 memory-actual ( RO): 268435456
                 memory-target ( RO): <expensive field>
               memory-overhead ( RO): 5242880
             memory-static-max ( RW): 268435456
            memory-dynamic-max ( RW): 268435456
            memory-dynamic-min ( RW): 268435456
             memory-static-min ( RW): 134217728
[root@XenGz cd]# xe vm-list params=uuid name-label=debian
uuid ( RO)    : 6e79ff87-6a67-6a07-325a-5cf7570281bb

[h5]Modify the memory:[/h3]

[root@XenGz cd]# new_mem=1073741824
[root@XenGz cd]# xe vm-param-set memory-static-max=${new_mem} memory-dynamic-max=${new_mem} memory-dynamic-min=${new_mem} uuid=6e79ff87-6a67-6a07-325a-5cf7570281bb

List the newly modified values:

[root@XenGz cd]# xe vm-list params=all name-label=debian | grep memory- | grep -v recommendations
                 memory-actual ( RO): 268435456
                 memory-target ( RO): <expensive field>
               memory-overhead ( RO): 11534336
             memory-static-max ( RW): 1073741824
            memory-dynamic-max ( RW): 1073741824
            memory-dynamic-min ( RW): 1073741824
             memory-static-min ( RW): 134217728

As you can see above and below, the memory-actual will be changed once the VM will start.

[root@XenGz cd]# xe vm-start vm=debian
[root@XenGz cd]# xe vm-list params=all name-label=gzvirt | grep memory- | grep -v recommendations
                 memory-actual ( RO): 1073745920
                 memory-target ( RO): <expensive field>
               memory-overhead ( RO): 11534336
             memory-static-max ( RW): 1073741824
            memory-dynamic-max ( RW): 1073741824
            memory-dynamic-min ( RW): 1073741824
             memory-static-min ( RW): 134217728