Modifying router.name and router.field after creating a collection

Question

i have created a collection in solr with number of shards=2 and replication factor=2 and while doing that by default solr assigned router.name=compositeId but I want to change the name to "implicit" and I also want to specify the value of router.field. I read somewhere that for doing that you have to edit the file clusterstate.json but I am not able to edit the file. Can somebody help how to edit this file or is there another way to change my router.name and specify the value of router.field?