At times you wish to change the sequence the computer boots up in. It normally looks at the C: drive first and if it finds all it needs witch is to say windows in the normal configuration, it reads that and is off and running.  However sometimes you wish to change the sequence so it will start somewhere else.  Typically the DVD drive or the a USB stick when you want to install a new windows or run some specialized software like certain anti virus software.  Keep in mind that a disk or USB stick has to be especially formatted for the computer to start off with it.

As a side note: There is a name change going on in the industry  BIOS (Basic Input Output System) is being changed to UEFI (Unified Extensible Firmware Interface) and the older system is being renamed Legacy.  Both BIOS and UEFI are the first thing loaded at startup.  They help the hardware and software communicate particularly before the hard disk has been read. UEFI has a number of security and other advantages over BIOS.  Some systems use both.  Macs have only used UEF for some time.  PCs just began supporting UEFIs with Windows 8 but machines made as early as 2012 have the capability.  Over time, BIOS will die out.

The fist thing to check is to Google your computer and model asking something like "How do I change the boot sequence on a Dell Insperon 11 3179 computer".  Many computers have a shortcut way of doing this which involves pressing some key reputedly while it starts up.  For example, on my Dell the trick is to keep tapping the F12 key while it is starting up.  Some other keys are DEL, F2, F8, F10 or ESC.

If your computer is a "home built model" try searching on the mother board.

The final approach is to go into the startup settings to change it.

There are a number of ways to get there.

One is to Newer computers using the UEFI system (Made after 2012 or so) assuming you have a windows 10 computer, click the start icon in the lower left of your screen then selecting "Settings", the gear icon. In "Settings" select "Update & recovery" (It is the last item in the menu)..  Now select "Recovery" about half way down on the left menu.  Now select "Advanced startup in the right menu at  the bottom" then "Restart Now" and the computer will start back up with a bunch of options and titled "Choose and Option.  If the second option is "Use a device" then select this to specify what device, i.e. DVD of USB drive, if "use a device" is not an option then you have a computer that has a special key to press during start up so Google your computer make and model to see what it is.

If your computer is so messed up it will not boot at all (will not load windows) then, you can still get to the UEFI settings by forcing the computer to shutdown by holding down the start hardware button on 3 or 4 startups and it will take you to Advance startup (Boot) Options instead of trying to load windows.

To see various advanced startup settings lick HERE.

A fairly good video that takes you from downloading a fresh version of windows to installing it, see https://www.youtube.com/watch?v=-eDX7f6EcLA

A shortcut for getting to the Troubleshoot menu is to hold down the shift key while selecting Start > Restart, when the new menu finally opens up on a blue screen, select Troubleshoot.  If the Troubleshoot menu has 23 options and the second one is "Use a device" then you are in free.  Just choose which device to boot from.  If you do not see that option when you are back to having to Google your computer to see what function key you have to press during start up.