During the creation of a VM, you can configure some parameters but not all parameters are fully configurable during the time of installation. This is where the default settings of Oracle VM Manager come into force, for instance assigning the VIFs (Virtual Network Interfaces), number of cores, and so on.
In order to modify the VM, you will click on the Virtual Machines page and in the VM table go ahead and click on Configure button. Alternatively, you can click on the VM itself to open the edit page of the VM:
On the edit page, you will be able to see the following tabs:
Some of the settings like increasing the memory, modifying NIC information, modifying the storage information, and so on, can take place immediately. For other settings you will need to bring the VM down and restart it for the changes to take effect.