I’m assuming you’re running the 3.0 release of ZS?
You could use the command line and something like dd to write the image manually to disk, but that can be problematic if you need specific drivers to recognize your HDD’s.
With packages there is the section On-line Updates https://www.zeroshell.org/patch-details/ – but I’m unsure which or if all of these are compatible with 3.0.
Personally the easiest way is to subscribe, in Zeroshell main screen click on Subscription button & make a note of your individual feature code, then visit https://www.zeroshell.org/activation/ which contains instructions on how to get an activation code (Requires some form of contribution & e-mail validation by Fulvio).
With the activation code you can then go back to Subscription, enter the key & save. Afterwards you’ll be able to download packages including Install Manager & others.