@PhilipDAthIs correct and this is probably the easiest way to do it.
A simple for loop through networks and ssids will get you through the data you need to change. Something like the below.
nets = meraki.organizations.getOrganizationNetworks(organizationId)
for net in nets:
ssids = meraki.wireless.getNetworkWirelessSsids(net['id'])
for ssid in ssids:
if ssid['name'] == "something":
ssid.update({'psk' : 'newpassword'}) # use the update() function when working with dictionaries. #
ssidChange = meraki.wireless.updateNetworkWirelessSsid(net['id'], **ssid) # the double * (**) will unpack the dictionary and allow you to post the full dictionary as the new variables. Be careful with this as you may get a return value that won't post.