If your boot manager is GRUB, then boot into Ubuntu and install GRUB Customizer (run a Google search for it). GRUB Customizer allows you to change the order of your boot menu, the amount of time before the computer automatically boots to an OS, which OS is the default, and much more.
If your boot manager is the Windows Boot Manager, then try using a program called EasyBCD. You can edit the Windows Boot Loader with that program.
If you get confused, you can search for help with using these programs on Google.