There are two different systems with the same model number -- one from 2011 or so (for which A16 is the latest update), probably your system) and one from 2022 (which is technically the Inspiron 15 3521, for which A12 is the most recent update).  It would appear you have the last update for the 2011 version, and do not need an update. 
I want to update the BIOS on this Inspiron 3521 Laptop but the installed BIOS is A16, 24/05/2018 - however , when I go to the Dell Support Site and look for a BIOS update for this laptop it says its 1.12.0 !  (i would have expected A17 or A18 etc..) One half of me wants to update the BIOS to get it up to date - the other part of me does not want to brick the laptop! 
Inspiron 3521 - conflicting BIOS updates