I have imac 27" 2011 can i update to 10.15
can i update to 10.15 i have high sierra 10,12
iMac 27″, macOS 10.13
A late-2009 through mid-2011 iMac can be upgraded to High Sierra 10.13.6 but no later version.
Also see the following from Identify your iMac model - Apple Support
iMac (27-inch, Mid 2011)
Model Identifier: iMac12,2
Part Number: MC813xx/A, MC814xx/A
Latest compatible operating system: macOS High Sierra 10.13.6