There is not a way to change the the priority. The Meraki MXs work based on the most-specific route model. With Static routing however, you may be able to "force" placing the AutoVPN route by creating a static route that is similar to the AutoVPN you want to make priority.
What is the situation and what are you trying to accomplish? I also attached the routing behaviors document below.
you can try to make it work by using bigger /overlapping subnets(supernets) to the MPLS.
Route priority dictates how traffic is routed when multiple routes exist to the same subnet. However, overlapping routes that are not identical are also present in many deployments. In this case, the most specific route will be used.
I dont know your ip plan or design but for example you could set static routes for all RFC1918 private ranges to the Mpls. As long as your vpn tunnels are up and more specific routes are learned there it would prefer the autovpn. (Did not test this . Assumption reading the doc.)