What I described is a way to bulk update, via scripting. Meraki's designed so that a lot of bulk changes are executed by the Dashboard API. Partly, I think, so someone can't bork up an entire organization's networks at once with a misclick.
It's a fairly simple script to dash off if you're familiar with interacting with an API and feeding data into a for-loop. In this case, you want a list of your network IDs and a variable with the JSON for your desired SNMP settings.
Even if you're not familiar, if you've got some programming experience, it should be relatively quick to pick up.