Where are you coming from? why do you want safe mode?
If you stuck outside windows, on another PC, download the
Windows 10 media creation tool and use it to make a win 10 installer on USB
change boot order so USB is first, hdd second
boot from installer
on screen after languages, choose repair this PC, not install.
choose troubleshoot
choose advanced
choose start up options << this may not be here
hit the restart button
choose a safe mode (it doesn't matter which) by using number associated with it.
choose start up options
hit the restart button
choose a safe mode (it doesn't matter which) by using number associated with it.
PC will restart and load safe mode
If you are on desktop, see option 2 here...