Go into the control panel and select system.
Once this opans click the advance tab and go to the startup and recovery settings.
From there you will be able to set the amount of time you allow for selecting the operating system.
The time you need to change is called time to display list of operating systems change to about 10 second to make sure that if you need to change again you can select the other operating system.
Pleas note my computer is set to 3 second to display the list and 30 to display recovery options when needed.
If you find the menu you will understand all of the above
Hope this helps