IMPORTANT !!!! READ THIS FIRST !!! Back up your important data before downgrade!!!
This video will show you how to downgrade Android 9 Pie to Android 8.0/8.1.0 Oreo.
If your phone has firmware build G960USQS4CSC3, G960USQU4CSC7, G965USQS4CSC3, G965USQU4CSC7, G960FXXS3CSD1, G960FXXS4CSDD, G965FXXS3CSD1, G965FXXS4CSDD, G9600ZHS4CSD3, G9650ZHS4CSD3, N950FXXU6DSD5 or higher you CANNOT downgrade
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Check you model number and firmware build inside your phone so you can download correct firmware for downgrade. (Go to Settings-About phone-Software information)
Here you can see a list of example firmware that can be used for downgrade to 8.0 for each model:
G950F 8.0 - G950FXXU4CRL3_G950FOXM4CRL1_BTU (UK)
G950N 8.0 - G950NKSU3CRI3_G950NOKR3CRI3_KTC (Korea)
G950W 8.0 - G950WVLU5BRG4_G950WOYA5BRG4_XAC (Canada)
G950U 8.0 - G950USQU5CRIB_G950UOYN5CRIB_SPR (Sprint USA)
G950USQU5CRK1_G950UOYN5CRK1_TMB (T-Mobile USA)
G950USQU5CSA4_G950UOYN5CSA4_VZW (Verizon USA)
G950U1 8.0 - G950U1UEU5CRK3_G950U1OYM5CRK3_XAA (USA)
G955F 8.0 - G955FXXU4CRL3_G955FOXM4CRL1_BTU (UK)
G955N 8.0 - G955NKSU3CRI3_G955NOKR3CRI3_KTC (Korea)
G955W 8.0 - G955WVLU5BRG4_G955WOYA5BRG4_XAC (Canada)
G955U 8.0 - G955USQU5CRG3_G955UOYN5CRG3_TMB (T-Mobile USA)
G955USQU5CRIB_G955UOYN5CRIB_SPR (Sprint USA)
G955USQU5CSA4_G955UOYN5CSA4_VZW (Verizon USA)
G955U1 8.0 - G955U1UEU5CRJ2_G955U1OYM5CRJ2_XAA (USA)
G9600 8.0 - G9600ZCU3ARG4_G9600CHC3ARG4_CHC (China)
G9600ZHU3ARJ1_G9600TGY3ARJ1_TGY (Hong Kong)
G9600ZHU3ARG4_G9600OWO3ARG4_BVO (Bolivia)
G9600ZHU3ARK3_G9600OWO3ARK1_TPA (Panama)
G960F 8.0 - G960FXXU2BRGA_G960FOXM2BRG8_SEE (South East Europe)
G960FXXU2BRJ3_G960FOXM2BRJ3_XSA (Australia)
G960FPUS2BRJ6_G960FSER2BRJ3_SER (Russia)
G960FXXU2BRJ8_G960FOXM2BRJ4_PHN (Netherlands)
G960FXXS2BRK3_G960FOXM2BRJ3_NEE (Nordic Countries)
G960FXXS2BRK3_G960FOXM2BRJ3_XEU (UK / Ireland)
G960N 8.0 - G960NKSU1ARG7_G960NOKR1ARG7_KOO (Korea)
G960U 8.0 - G960USQU3BRJ5_G960UOYN3BRJ5_SPR (Sprint USA)
G960USQU3BRJ5_G960UOYN3BRJ5_TMB (T-Mobile USA)
G960USQS3BRJ6_G960UOYN3BRJ6_VZW (Verizon USA)
G960U1 8.0 - G960U1UEU3ARK5_G960U1OYM3ARK5_XAA (USA)
G960W 8.0 - G960WVLU3ARH3_G960WOYV3ARH3_XAC (Canada)
G960WVLS3ARJ6_G960WOYV3ARJ6_XAC (Canada)
G9650 8.0 - G9650ZCU3ARG4_G9650CHC3ARG4_CHC (China)
G9650ZHU3ARK1_G9650TGY3ARK1_TGY (Hong Kong)
G9650ZHU3ARG4_G9650OWO3ARG4_COO (Colombia)
G9650ZHU3ARK3_G9650OWO3ARK1_TPA (Panama)
G965F 8.0 - G965FXXU2BRG6_G965FOXM2BRG4_DBT (Germany)
G965FXXS2BRI1_G965FOXM2BRG8_XTC (Philippines)
G965FPUU2BRJ3_G965FSER2BRJ3_SER (Russia)
G965FXXU2BRJ3_G965FOXM2BRJ3_XSA (Australia)
G965FXXS2BRJ6_G965FOXM2BRJ3_NEE (Nordic Countries)
G965FXXS2BRK3_G965FOXM2BRJ3_BTU (UK)
G965N 8.0 - G965NKSU1ARG7_G965NOKR1ARG7_KOO (Korea)
G965U 8.0 - G965USQU3BRJ5_G965UOYN3BRJ5_SPR (Sprint USA)
G965USQS3BRJ6_G965UOYN3BRJ6_VZW (Verizon USA)
G965USQS3BRK2_G965UOYN3BRK2_TMB (T-Mobile USA)
G965U1 8.0 - G965U1UEU3ARK5_G965U1OYM3ARK5_XAA (USA)
G965W 8.0 - G965WVLU3ARH3_G965WOYV3ARH3_XAC (Canada)
G965WVLS3ARJ6_G965WOYV3ARJ6_XAC (Canada)
N950F 8.0 - N950FXXU5CRK2_N950FOXM5CRK1_BTU (UK)
N950N 8.0 - N950NKSU4CRJ2_N950NOKR4CRJ2_KTC (Korea)
N950U 8.0 - N950USQS5CRJ3_N950UOYN5CRJ3_SPR (Sprint USA)
N950USQS5CRJ3_N950UOYN5CRJ3_TMB (T-Mobile USA)
N950USQS5CRK1_N950UOYN5CRK1_VZW (Verizon USA)
N950U1 8.0 - N950U1UEU5CRJ2_N950U1OYM5CRJ2_XAA (USA)
N950W 8.0 - N950WVLU5CRJ3_N950WOYV5CRJ3_XAC (Canada)
N9600 8.1.0 - N9600ZCU1ARJ6_N9600CHC1ARJ6_CHC (China)
N9600ZHU1ARK1_N9600OWO1ARK1_TPA (Panama)
N9600ZHU1ARL1_N9600TGY1ARL1_TGY (Hong Kong)
N960F 8.1.0 - N960FXXU2ARJ1_N960FOXM2ARJ1_PHN (Netherlands)
N960FPUS2ARK1_N960FSER2ARK1_SER (Russia)
N960FXXS2ARK1_N960FOXM2ARK1_XSA (Australia)
N960FXXS2ARK2_N960FOXM2ARK2_BTU (UK)
N960FXXS2ARL3_N960FOXM2ARK2_SEE (South East Europe)
N960U 8.1.0 - N960USQU1ARJ5_N960UOYN1ARJ5_VZW (Verizon USA)
N960USQU1ARJ8_N960UOYN1ARJ8_TMB (T-Mobile USA)
N960USQS1ARJ9_N960UOYN1ARJ9_SPR (Sprint USA)
N960U1 8.1.0 N960U1UEU1ARJ5_N960U1OYM1ARJ5_XAA (USA)
N960W 8.1.0 N960WVLU1ARJ5_N960WOYV1ARJ5_XAC (Canada)
