Best software upgrade for iMac 21.5" late 2009

The App Store Updates page only informs you of updates available for the OS you are currently running.

As you are running an older system then you are probably up to date on that system.

If you want to install a newer OS that is called an Upgrade. 

The model and year of your Mac will determine which macOS you can upgrade to,

read the following links.

How to upgrade to macOS High Sierra - Apple Support

Before upgrading make a full back up of your Mac.

Check that third party apps you use have updates available for the new OS.

https://roaringapps.com

Check that your printers/ scanners etc have updated drivers and software available for the new OS.

macOS High sierra is as far as your mac can upgrade to.

You have two posts on the go with the same subject.

If you have an external drive that is free to use, i.e. empty you can use that to back up your mac.

Plug it in to your mac, open Disk Utility (Applications/Utilities), in the left column highlight the

Disk not the indented Volume, click Erase. Give the Disk a name,

Format: Mac OS X Extended (Journaled),

Scheme: GUID Partition Map, click on Erase.

After that has completed open System Preferences> Time Machine> Select Disk

choose your newly formatted external disk. Click Options and here you can add

things you do not need backed up. As this is your first back up you may want to

include everything so don't bother with this bit. Checking the Back Up Automatically

button will give you a count down to when the back up will start.

With automatic back ups on the mac backs up every hour.

If you just want a one off back up, don't check the Auto button but check the

Show Time Machine button in the menubar. A clock icon will show, you can click

on that and press Back Up Now.