how do i update iMac late 2009

Any updates will be found if available via the App Store Update page.

If you actually mean upgrade, i.e moving from one OS to a newer OS

then your 2009 iMac tops out at Mac OS X El Capitan.

if you have not installed Mac OS X El Capitan it can be downloaded from here,

How to upgrade to OS X El Capitan – Apple Support

Go to Section 4 and click on Get OS X El Capitan.

If that is what you are going to do then you can ask me for further instructions.