@sufurd When your phone is turned off (fully powered down), press volume up AND power button simultaneously. Keep pressing until the Android recovery menu has appeared and then release the buttons.
Now, in the recovery screen, use volume up and down to move in the menu, select Wipe data/factory reset. When you’re on that menu, use the Power button to Confirm / select that option. Then select: Factory data reset.
Once confirmed, the process will take several minutes and once complete, select: Reboot now
If the Recovery screen doesn’t appear, check that the buttons are in fact working correctly.