Bios download link as below:
for BIOS Ver A18 : https://downloads.dell.com/FOLDER04976362M/1/L421XA18W.exe
for BIOS Ver A17 : https://downloads.dell.com/FOLDER04818333M/1/L421XA17W.exe
for BIOS Ver A16 : https://downloads.dell.com/FOLDER01915451M/1/L421XA16W.exe