Hi twobob, and welcome to our forums
It's an interesting question you raise and here are some thoughts that may help.
1 If you have not already run the Windows 10 compatibility ( to check that your machine can run Win 10 Ok) then this would a wise first step
2 Before making fundamental changes to your computer it would be wise to make some disc images Plus a repair disc using an application like Macrium Reflect free or other software so that in the event of serious trouble you could put the computer back to how it was before you started, so to speak. 3 copies of the images, on different media are often felt to be sufficient as long as they have been verified as accurate and reproducible and at least one verified copy is kept safely offsite on removable media.
3 If the original OS is Win 7 for example then it would be wise to use the native Windows tools to make a repair disc and some system images (in addition to those at step 2 above)
4 To boot from a flash drive, it will probably be necessary to enter the BIOS and set the boot priority to
1 USB flash drive
2 Hard drive 0
(the rest don't really matter as long as the first two are correct)
Save the changes, close down your computer, insert your Flash drive (that is hopefully bootable) and restart your computer.
5 It should now boot to the flash drive and start loading Windows files
6 Before you start I would check that you have a correctly licenced Win 10 operating system.... post back if you need extra information on that