There are some ways, this is simple way how to do...
Method 1
Follow this steps:
1. Pull the battery out
2. Hold both volume up and volume down
3. Hold Power button
4. Connect your usb to a PC and phone
5. Insert your battery
6. Flash rom using odin
By using samsung kies or samsung kies 3
This methode is very good way to repair and flash, or factory reset samsung phones. With samsung kies you will find an option emergency device and software initialization, click on it and then put your samsung model number, with capital letters, and to SN put serial number of you phone. Serial number is back to the battery of your phone. Click next and let program to do other part of job. Do not disconnect your phone until process is finished. And after process is finished you are ready to use your phone or tablet.
By using Smart Switch
Smart switch is a software made sy samsung to support some devices like galaxy s7 etc. This is a very good software to make a device syncronisation, reset, and flash your phone or tablet. This software allows you to access your files wich computer, photos, contacts, music etc.
This software is avariable for windows and mac computers.
Flash samsung using smart switch - when you open program you will see connect your device, and another option to right to more. click on more and select "emergency firmware and device initialisation" and then put model number and serial number, after that click next and start flashin.
CAUTION : All your data will be deleted from your device tablet or phone.