upgrade macOS high sierra

A late-2009 iMac can be upgraded to High Sierra 10.13.6 but no later version.

See the following from: Identify your iMac model - Apple Support

iMac (21.5-inch, Late 2009)

Model Identifier: iMac10,1

Part Number: MB950xx/A, MC413xx/A

Latest compatible operating system: macOS High Sierra 10.13.6

How to get old versions of macOS and verify what version this computer Qualifies to install. Make sure to use Only Safari to commence the download as Others may not work

Make a Time Machine Backup Before attempting.