There are two ways to install the downloaded firmware.
Ensure the phone has at least 70% battery to prevent bricking during installation. Oppo F1s Firmware Download 7-1-2
Solution: The zip file may be corrupted or for the wrong model. Re-download the exact firmware for your A1601/A1603. Also, ensure the file is not renamed incorrectly. There are two ways to install the downloaded firmware
Flashing custom firmware will erase all user data (apps, contacts, photos, messages). Backup to Google Drive, PC, or an external SD card. messages). Backup to Google Drive
If your phone hasn't been formatted recently, the update might still be cached in the system.