The easy way to do this is by downloading and installing EasyBCD
from Download EasyBCD 2.1.1 - NeoSmart Technologies
(don't let the "buy it" button distract you, you can download it for free at the bottom of the page).
EasyBCD manages the boot manager entries and allows you to choose an OS as the default boot option, decide whether you want the boot menu shown at all, and if yes, set how long it will wait for a user choice before booting to the selected default.
If you prefer not to download a third-party app and
you're comfortable with using the bcdedit
command in a DOS window, I'll be happy to help you along with that instead.