Since Samsung’s official servers often archive old files, these community-trusted databases are your best bet:
Uses a different processor; firmware is not compatible with the standard I9100. SGH-I777 / SPH-D710: US carrier versions (AT&T/Sprint).
This blog post provides a safe and clear guide for users looking to download firmware for the classic Samsung Galaxy S2 (GT-I9100). Since this is a legacy device, finding reliable files is key to avoiding "bricked" phones.
Once you have your firmware file (usually a .tar or .tar.md5 file), you will need these tools to install it:
The official-standard leaked tool for flashing Samsung devices on Windows. (Version 3.07 or 3.09 is usually best for older devices like the S2).
Search for your model (e.g., GT-I9100 ) on one of the sites above. Select your (the "CSC" code).