It's a bit difficult for me to assess the extent of the concaving from the photos. If it is an old build, and your climate is dry, rehydration might help.
With a new build it is likely that the relative moisture content of the back wood and brace wood was significantly different on glue up. When this happens the cross-grain shrinkage of the back wood pulls the braces into an arc.
The last time I had one like that, nothing helped: It was back off, chisel off the braces, reduce back and new brace material to 40% RH and reglue. This is relatively simple when the instrument is not bound.
I hope rehydration is the answer in your case.
Wait for other opinions before you try anything, others might have had more remedial successes than I have.