how to update to mac os 10.15

A late 2009 iMac cannot be upgrade beyond High Sierra 10.13.6.

Also see the following from Identify your iMac model - Apple Support

iMac (27-inch, Late 2009)

Model Identifier: iMac10,1

Part Number: MB952xx/A, MB953xx/A

Newest compatible operating system: macOS High Sierra

iMac (21.5-inch, Late 2009)

Model Identifier: iMac10,1

Part Number: MB950xx/A, MC413xx/A

Newest compatible operating system: macOS High Sierra

That Mac model can't be upgraded past High Sierra.

(245908)