Usually by holding power/volume up and sometimes the home button.
Hold until the device reboots, keep holding and a recovery menu should open.
You can use the volume down and up to navigate the menu, and power to select.
Look for factory reset.
Note: you may need your google account info to finish reactivating the phone after reseting the phone in this way. You could get locked at that stepped without it.
Other note: button combo is different for all devices. Do a quick search on google for your model and hard reset.
I hope this helps