what is the latest update I can do on a iMac (retina 5k, 27-inch, Late 2015

Monterey…

macOS Monterey is compatible with these computers - Apple Support

• iMac – all models from late 2015, iMac 16,1 – 20,2
• iMac Pro – all models, iMac Pro 1,1
• MacBook & MacBook Retina – all models from early 2016, MacBook 9,1 – 10,1
• MacBook Pro – all models from early 2015, MacBook Pro 12,1 – 17,1
• MacBookAir – all models from early 2015, MacBook Air 7,1 – 10,1
• Mac Mini – all models from 2014, Mac Mini 7,1 – 9,1
• Mac Pro – all models from 2013, Mac Pro 6,1 – 7,1

https://wpbeaches.com/apple-minimum-system-requirements-for-macos-monterey-12/

https://apps.apple.com/us/app/macos-monterey/id1576738294

Ventura is out of reach for te Late 2015 iMacs...

The following Mac computers are supported by macOS Ventura:

MacBook (2017 and newer)

MacBook Air (2018 and newer)

MacBook Pro (2017 and newer)

Mac mini (2018 and newer)

Mac Studio (2022)

iMac (2017 and newer)

iMac Pro (2017)

Mac Pro (2019 and newer)

macOS Ventura system requirements: Can your Mac run macOS 13? (idownloadblog.com)