My mac won't update

What system are you running now? If it's an earlier version of Ventura then try the following:

Boot into Safe Mode according to How to use safe mode on your Mac and try installing from there.

Note:  if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode.  This makes it act as a wired keyboard as will assure a successful boot into Safe Mode.

Give this a try: boot into Safe Mode according to How to use safe mode on your Mac and test to see if the problem persists. If you're successfull install from the Safe Mode.

NOTE 1: Safe Mode boot can take up to 3 - 5 minutes as it's doing the following; 

• Verifies your startup disk and attempts to repair directory issues, if needed

• Loads only required kernel extensions (prevents 3rd party kernel/extensions from loading)

• Prevents Startup Items and Login Items from opening automatically

• Disables user-installed fonts 

• Deletes font caches, kernel cache, and other system cache files

NOTE 2: if you have a wireless keyboard with rechargeable batteries connect it with its charging cable before booting into Safe Mode. This makes it act as a wired keyboard as will insure a successful boot into Safe Mode.

Whether, and how far, you can update a Mac will depend on its model and hardware model year / time of year. You can often find this information by selecting About This Mac in the Apple () menu.

macOS Ventura is compatible with these computers - Apple Support

How to download and install macOS - Apple Support

I would add that you haven't given us any useful information to evaluate and then respond to. Please be specific about what happens. What happens when you try to upgrade or update?

 Writing an Effective Apple Support Communities Question - Apple Community

I too have 2 iMacs (18,3) with Ventura 13.7 that fails to show available updates in System Settings.

While using the Terminal "softwareupdate -l" command, the "No new software available" message appears.

The update is shown when I use "softwareupdate -l --list-full-installers" but trying to manually install it fails.

The "softwareupdate --fetch-full-installer --full-installer-version 13.7.1" command gives me this message:

"Scanning for 13.7.1 installer

Install failed with error: Update not found" 

However, the Safari updates show up in System Settings and can be installed normally.

Anybody has an idea about whats causing this?

Thanks