how can update my mid 2010 iMac
how can I update my macOS Sierra mid 2010
MacBook Pro 13", macOS 10.13
macOS High Sierra 10.13 Technical Specifications
General Requirements
OS X 10.8 or later
2GB of memory
14.3GB of available storage to perform upgrade*
Some features require an Apple ID; terms apply.
Some features require a compatible Internet service provider; fees may apply.
Mac Hardware Requirements
For details about your Mac model, click the Apple icon at the top left of your screen, choose About This Mac. These Mac models are compatible with macOS High Sierra:
MacBook (Late 2009 or newer)
MacBook Pro (Mid 2010 or newer)
MacBook Air (Late 2010 or newer)
Mac mini (Mid 2010 or newer)
iMac (Late 2009 or newer)
Mac Pro (Mid 2010 or newer)
https://support.apple.com/kb/SP765?locale=en_US
https://itunes.apple.com/app/macos-high-sierra/id1246284741?mt=12