You can't easily, they definitely were slipped in from the back. You now need to gut behind the stairs to gain access. This is honestly going to be a major pain in the a** to do properly if you are asking these questions now.

Screw and glue 2x2 horizontal supports to the stringer on either side for the end of every new treads to rest on. You won't have a center support but it looks like you never had one. 1" thick Oak tread will be able to bear the load but may flex and squeak a bit.