There's no way to do this within the UI once an EA has been activated. We lock this function down because the EA could be consumed on objects and those objects rely on the distinct flag for specific behavior, like formulation calculations. You have to use a database script to remove the distinct flag. I'll have to defer to dev for an exact script, but it should be pretty straight forward. Before making it non distinct make sure the EA isn't being used for formulation calculations or being relied on for reports etc.