This is a use case for a custom mutation backed by a Lambda that does this with DQL. Using DQL, you can read a value and perform a math operation and set the value all in a single transaction using upserts. The auto generated API aides with all simple CRUD operations, but what you are describing is business logic.
For this functionality, it might be easier to add an edge from the post to users who like the post. Then when a user likes a post it is a simple mutation to set the user on the edge. If it is already there no action will be taken. When a user unlikes a post it is a simple mutation to remove themselves from the like edge. . You can still query and subscribe to the count of the aggregate edge on this as well.