Go into settings and power menu. Tick screenshot.
Go into settings and about phone. Keep tapping on the build number until you have developers option.
Go into settings and developer's options and enable advance power menu.
Now you can hold the power button to take screenshots. G
Edit: sorry i have another question.
Do you guys know how to change the lock screen? I like it everytime i reboot my phone it comes out with the wallpaper i normaly have in my screen. But then after i lock my phone again it comes out with this snow mountain and a 24 hour format clock. I tryed everything but it dosent change