You can reboot in a number of ways such as clicking the icon in the top right corner of Ubuntu and choosing to restart or by using the reset option from the VirtualBox menu.
Ị nwere ike reboot n'ọtụtụ ụzọ dịka ịpị akara ngosi na akuku aka nri nke Ubuntu ma họrọ ịmalitegharịa ma ọ bụ site na iji nhazi nhọrọ site na menu VirtualBox.