The way I used to recommend doing this doesn’t appear to work well under the new architecture. But both @csstup and @HalD have done this successfully, using different Z wave sticks and somewhat different methods, so it can certainly be done. But it’s not easy and requires significant technical understanding.
In one approach, you add the stick to the SmartThings hub as a secondary controller. The problem there is you have to figure out exactly which device IDs you want to update, and that isn’t always obvious. Also, it can take a very long time, depending on the security levels, and that can cause some havoc with other actions on the network. So while I used to recommend that method, I no longer do except in a few specific cases.
The other approach is to remove the device to be updated from the SmartThings network, and then add it to the Z stick as a primary and then update it there. This has the advantage of not interfering with any of the other activity on your SmartThings network, but it does mean you would need to later re-add the updated device to SmartThings and redo any routines or associations. That’s a lot of time and trouble. And, of course, if the device to be updated is a repeater, you might create some network issues there while the work is being done.
In both approaches, you will need to use update software on a PC. You can’t really do it on a mobile device.
So… in most cases, in the SmartThings environment I would consider this a task for a field tech, not an end user. Obviously, there are many power users who can do this themselves, but that’s a different group.
Here’s one discussion thread that’s pretty current:
Leviton Dimmer Switch constantly going offline after recent update - #19 by csstup