I have an iMac (27" late 2012) with OSX Yosemite 10.10.5. Is there any system update available for this iMac?

Back up your data, check your applications for compatibility, click here, and then on the download link for a newer OS. Its maximum is Catalina.

Catalina and newer won't run 32-bit software; download and use Go64 as well as using the first link above.

(230242)

A Late 2012 iMac can run up to macOS 10.15 Catalina.

To update see > Update macOS on Mac - Apple Support