You can enable the legacy boot menu by opening an administrative command prompt (Right click the Start button > "Command Prompt (Admin)" ) and running the following command:
bcdedit /set {default} bootmenupolicy legacy
Next time you restart the system, press F8 repeatedly until you get the menu with the option to enter Safe Mode.
Edit:
You can also access it by going to Settings > Update & Security > Recovery > click Restart Now under Advanced Startup. The bcdedit method above is permanent (in that it will persist across restarts) though, so if you ever have problems getting into Windows, you can access the menu without running into the catch-22 of having to get into Windows to access the menu when you can't get into Windows.