Retrofitting Rusted Deck Standoff
There are several versions of that type of bracket available.
It is called a post base bracket.
My local home improvement centers carry them.
Here is one
It is fastened to the concrete with nut and bolt.
You may have problem when you remove yours as the nut and bolt are probably rusted like the bracket.
I would spray them with PB Blaster several times over a couple of days, heat it with a torch and wack the top of it with a hammer before you try to remove the nut.
If you break the bolt off you have a problem dealing with a steel bolt in the exact spot you need to drill for installing a new bolt/anchor. It may be easier to relocate the post if that happens.
You will need a temporary post to support the structure while you remove the old post and replace the bracket.