Increase Memory while running trial OVF

I have downloaded and testing the Messaging Gateway Small Business as virtual appliance. I have completed the installation using the OVF file and everything is working fine. The memory bu default is 4GB. I am now in the process of redirecting all of the emails to pass thought the gateway but I would like to increase the memory first as it currently shows only 150 MB free. Do I just increase the memory from VCenter or do I have to do something else?

Thanks.