When Ubuntu is the only operating system of the computer, you can remove it by using an operating system install disc for any operating system. Once you’ve inserted it, restart the computer and boot from CD, as outlined in Step 2 above. Once you start the installation process for the new operating system, you are given the opportunity to create and delete partitions on your hard drive. Select your Ubuntu partition and delete it. This will return the partition to unallocated space. Once the partition is deleted, Ubuntu has been successfully removed from the computer. You can now install a new operating system such as Windows 7 or Windows 8. If you choose not to install the operating system, your computer will be unusable until one is installed.

TL;DR in Vietnamese:
Chạy đĩa của hệ điều hành bạn muốn cài đặt trên máy tính. Xóa phân vùng Ubuntu. Tiếp tục cài đặt hệ điều hành, hoặc lấy đĩa ra và tắt máy.