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How to install a custom os

Hello i have a s3 i900 whitch seems to have a currupted os. When turned on regualry it displays a white screen with colorful Pixels somtimes. It can no go into revovery mode but it can go in the customs os downlaod mood with comfirms the screen is not the issue. Can anyone point me in the direction that i can reinstall the os on this s3 it would be really helpfull.

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