I think Kyu means, structurally in the Meraki dashboard. Does each site have a separate NETWORK in the dash for its wireless devices?
If so, easy enough. Just change the password(s) for the SSID(s) on that site's network.
If you're using a single wireless network for all sites, then you can't change the password on a specific SSID without changing it for everyone.
I'd also recommend splitting it out so you have one wireless network per physical site. Meraki trainers have told me that for appliances/switches/wireless, you want to ensure that each site has its own network.