Upgrading RAM in iMac (all system info shown)

I'd like to increase the amount of RAM on an iMac. The "About This Mac" system info is pictured below. It states the memory can't be upgraded.

I'm wondering if the CURRENT memory can't be upgraded because both slots are in use (?) or if this system won't take more than 8GB of RAM? (I hope that makes sense.) I guess the core question is what is the maximum amount of RAM an A1418 will take...?

My goal would be to remove the existing memory and replace the two 4GB RAM chips with 8GB RAM chips, but I'm seeing conflicting info.

Any and all assistance appreciated...system info posted below.

(New to this forum - couldn't find a post with this specific issue - apologies for stepping outside SOP, if I did.)

Thanks in adcance,

Eric