Sorry, but, according to Dell documentation, 8 GBytes is the maximum amount of memory a 15z supports.  The Crucial memory selectors also support that limit. 

Even if we upgrade the BIOS ,8GB would be the max RAM it can accomodate ? 

I already have 6GB RAM installed. How many DIMM's does it have ? what capacity RAM should i buy to increase it to 8GB ?

Thanks in advance