Can I upgrade from Big Sur 11.6 to Big Sur 11.7.6 in one download?

To get the latest Big Sur Update, go to Apple > System Preferences > Software Update, then wait until you see more shows up in blue under the Ventura Upgrade.

The other option, is to just go ahead and Upgrade to Ventura.

This seems to be a Mac that needs to be reset before use.

Here is how:

Erase your Mac and reset it to factory settings - Apple Support

Here is what the seller is supposed to do:

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

If you don’t erase and re-install, you will have issues with app updates and other details, as the macOS install and other apps installed will all be tied to an Apple ID that is not yours. And a Mac that may well be locked to a different Apple ID via Activation Lock, which tends to end badly.

Paul NYC wrote:

I'm pretty sure that the original owner had this done, but I will double check to make sure that it was done before I attempt anything. Thank you for pointing this out!

There’s normally a setup procedure during first boot, where you’ll enter info such as country, language, your name, password, Wi-Fi, etc.

This stuff: Set up your iMac - Apple Support

If that sequence happened, your install is yours, and you can get a backup, and upgrade to current Ventura.

Paul NYC wrote:

That didn't happen when I first turned on the computer. The original owner did give me his password, so I'll have to do the factory reset myself. Thanks again.

Yeah; if that didn’t happen, you’re headed for “fun”. The owner will also have to relinquish Activation Lock—which can be done remotely, see previous links—or you’ll have another “fun” awaiting.

When you click on the Apple logo then about this Mac and software update what does it say? There may be some incremental updates you can install besides going to the next OS upgrade. However, should be no issue you moving up to Ventura but just be aware of the new user interface which is different from Big Sur or Monterey.

If it were me I would just upgrade to Ventura. Make sure you have a backup first.

You're welcome.