1.first of all you need to click on the computer icon(This PC) right button of your mouse:
2.Move down to properties section:
3.now a window will open where you have to click on “advance system settings”
4.another window will open where you have to click on the third settings from the above.shown in the image
5.now you will see a new window with options of changing the delay time of displaying list of operating system.
You may increase displaying time of your operating system list or decrease it.