This is what worked for me on 2 computers that needed to revert back to Windows 7 from Windows 10
Start the computer up: When its booted into Windows but before it shuts itself down, hold the power button until the computer shuts down.
--- Repeat the same procedure a second time.
On the third startup, you should be able to boot into Advanced Startup Options
--- On one of the 2 computers I dealt with, one of them took 4 hard shutdowns instead of 2
How to Boot to Advanced Startup Options in Windows 10
"In some situations, Windows RE automatically tries to repair the system. Also, these situations automatically cause a failover to Windows RE:
Two consecutive failed attempts to start Windows.
Two consecutive unexpected shutdowns that occur within two minutes of boot completion."
--- There are some very good diagrams on how to proceed