Just press and hold the power button for like 10 seconds and that's it.
Some Lumias seem to have a one button soft boot feature.
I have a T-Mobile 635 here (under W10 currently)
I've always rebooted this phone by holding the screen off button for about 8-10 seconds.
If I hold the button then swipe down to shut off the phone off I see the "Goodbye" followed by the calendar reminder (W10 only) phone then stays off.
If I hold the V- and power button together I get the slide down for off.
Continuing for 10 seconds or so the screen blacks out phone vibrates and restarts.
If I hold the power button alone the slider shows up.
Continuing to press for 8 seconds the screen blacks out then vibrates and restarts.
This was the same under W8.1 Cyan.
When I first got this phone there was in the Lumia help and tips app a listing of one button "soft" reset devices.
I'm unable to find this information since the app has been changed.