Linksys EA8300 Direct Rom Flash Guide:

factory.bin - Flash from Stock Linksys Firmware
1. Go to http://192.168.1.1 → Manual Configuration → login (admin/admin)
2. Go to Smart Wi-Fi Tools → Connectivity → Manual → Upload
3. Wait ~3 minutes (device reboots twice automatically)
4. Done! Connect to LAN1 port (10.0.0.1)

sysupgrade.bin - Firmware Upgrade via LPB Admin Panel
Go to http://10.0.0.1/admin/dashboard/maintenance → Firmware Upgrade

No USB drive needed - everything is baked into the ROM.