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You will need ODIN, Found at: https://odindownload.com/ Odin3.09 is the more stable version.
You will need Your Required Firmware, Found at: https://www.sammobile.com/ You will need to sign up, and log in to download firmware from this site. it is all OEM stock. this site is not for custom ROMS.
If your ROM is not located here, you can try to find it at https://forum.xda-developers.com/. this place also does custom ROMS. (PLEASE BE ADVISED: Custom ROM work, if done improperly, can cause failures in device. DO NOT use this source if you do not have experience with software reloads )