I think it is locked Down by default in sharepoint 2013. As a workaround, you can make a copy of that result source the modify it.

I think it is locked Down by default in sharepoint 2013. As a workaround, you can make a copy of that result source the modify it.

You can point to the arrow next to an existing result source, click Copy, and then modify the copy as necessary and save it with a new name.