We tried to edit the default “Subject view” for Discussion Board list in SharePoint 2013, but Modify view is in Disable mode option.
If we want to enable this option, go to "SharePoint Designer" open the Particular site and list that you want to enable the view. Click on the Subject View
In the Designer we can able find the code ReadOnly = “True”, change it to “False”.
Click on save.
Now we can able to see the option to modify the view in the SharePoint GUI.