Upgrading this switch requires a specific "stepping stone" approach to avoid an "Illegal software format" error. SG200-26 26-Port Switch Firmware upgrade - Cisco Community
sx200_boot-13506.rfb (required for newer firmware compatibility) Critical Upgrade Path sg200-26 firmware download
Cisco SG200-26 (Small Business 200 Series) End of Service Life (EOSL) Upgrading this switch requires a specific "stepping stone"
This is the most critical step. Downloading the wrong firmware will brick your switch. sg200-26 firmware download