Two things -
Soft reset (Volume Down + Power)
It'll just restart the phone.
Hard reset (restart the phone, and while it boots up, hold down the Volume Down key to get an exclamation point to show up. Once it does, press, in order, Volume Up, Volume Down, Power, Volume Down).
This will erase all personal data so you can set up your phone like it was new, just on the current build of Windows 10.
Hope one of those helps, and good luck!