Bulk Updating Entities

This can be done very quickly with a simple flow! 
You will first want to use a Fetch Entities Step that fetches each case entity you would like to update. 

You will then want to add a ForEach Step. This will allow you to run a Set Entity State step on each item you fetched via the Fetch Entities Step. Within the ForEach Step you will want to set the Collection to your EntityResults (the Fetch Entities output) and set the Data Type to the case entity you are using. 

Within the Set Entity State step, you will want to map in the Entity ID as the Item.EntityID. This will take the ID of the item that is currently being used in the ForEach Step. You can then set the New State to whatever you would like it to be.