Open vagrant.yml file and edit respective values.
v.gui- Set totruefor debugging. This will unhide VM's primary console screen. Default:false.v.memory- Memory settings (MB). Default:2048.v.cpus: 1- number of virtual CPU cores. Default:1.
Please note, VirtualBox works much better with a single CPU in most cases, this it is not recommended to change the v.cpus value.