Is upgrading the SSD possible?

If you are talking about swapping out the 3.5" HD for a 2.5" SSD you'll have the needed Custom SATA cable in place, otherwise you'll need to pop the logic board out to install the cable which can be gotten as a spare.

In addition! You still need the OWC in-line thermal sensor as well as a 2.5" to 3.5" adapter frame: OWC 2.5" to 3.5" Drive Adapter Bracket.

If the system has a Fusion Drive, you'll need to break it BEFORE you attempt any alteration and you CAN'T create a Fusion Drive set using two SSD's!

As for upgrading the Apple custom blade SSD, sadly there is no 3rd party upgrades presently. Your only choice is to get a REAL Apple blade SSD. But these are very expensive! In addition your system has a custom Apple PCIe x4 NVM Express SSD. It also requires a lot more work to get to.

Even opening the 'Thin Series' is a bear! Here's the IFIXIT guide: iMac Intel 27" Retina 5K Display Replacement. Make sure you follow the guide to the letter and use the correct tools! If you don't you can kill the display!