Your 2009 iMac can only upgrade to El Capitan.
How to upgrade to OS X El Capitan – Apple Support
Go to Section 4 and click on Download OS X El Capitan.
This downloads InstallMacOSX.dmg to your Downloads folder.
When downloaded open to InstallMacOSX.pkg, double-click on
that and an installation window will open, this does not install El Capitan
but converts the .pkg to the Install OS X El Capitan.app which you will find in
your Applications folder, it should be 6.2GBs in size.
To start the installation of El Capitan double click on the Install OS X El Capitan.app.
The copy of the install app self deletes after installing El Capitan, so make sure you keep a copy of the InstallMacOSX.dmg
if you need it in future, or you could just make a copy of the Install OS X El Capitan.app prior to installing and moving it to
an external drive for safe keeping. It is also possible to create a bootable USB installer disk, using the Install OS X El Capitan.app
in the Applications folder, and the createinstallmedia command in the Terminal app. Read the instructions here,
How to create a bootable installer for macOS – Apple Support