1. Take a look what android firmware is using (Settings - General - About phone)
2. Go to DOWNLOAD MODE (Power off ur phone + Volume down. Volume up then. - Take a look how is the KNOX)
3. Install Kies (if ur phone use android 4.2.2) or Kies 3 (android 4.3 or superior) and take a look if all is ok ( Important see what CSC u use). Only making a factory reset solve the problems sometimes(Settings > Accounts > Back Up and Reset > Factory Data Reset > Reset Device > Delete All/Erase Everything). If this dont solve ur problems, follow the next step
4. Your seller installed one custom rom maybe. If all is ok in the step 3, you can update ur phone by there officially. If no, you will have to use ODIN and flash new stock rom.
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