July 29 is the official deadline for installing Windows 10 upgrades free of charge.
I put off the upgrade until very late, until yesterday, July 11, in hopes that by now the installation bugs have been minimized.
Yesterday I got down and dirty and installed Win 10 on two desktops running Win 7 Pro, and on one running Win 8.1.
My experience with this leads me to give two hints.
[1] Before starting the upgrade process, disconnect all easily disconnectable peripherals like external drives, USB sticks.
If you have a desktop with a graphics card or a sound card that was added to the motherboard after the computer was purchased, cross your fingers. It might be possible to install Win 10 without removing these. But if problems persist you might have to uninstall these, first uninstalling the drivers, and then second removing the cards. Groan !
[2] If at all possible, have a second computer up and running while you do the upgrade: it makes a huge difference, because when an installation error message pops up, you can google on the second computer to find the fix, without interrupting the install process. Having the second computer is not absolutely necessary, but it helped preserve my sanity, in coping with several error messages.