Slip a pipe over the bar at the top of the jack and crank the jack to level.

And then get a concrete drill and put in some bolts so that the unsecured posts stay in place, particularly if you are anywhere an earthquake could occur. What you have there can't possibly meet code regardless of where you live.

Do I need to install more 2x6 in the floor of the Bay Window to the interior of the basement? (See the last picture of 2x6 support to the interior of the basement.

Are those only 2x6's? How much cantilever?

I think you need a combo approach. Gradually jacking it up (like Joesph said but don't do it all at once) and then sistering the joists a good ways back into the basement. (You will need to remove the bottom sheathing on the exterior of the bay.) There will be a prescribed fastening schedule that you need to adhere to. You will probably be able to dig all that up on the web somewhere...or consult with a structural engineer.

Joseph, thanks for the recommendation.

Kudzu9. How do you know the the window is out of code? What will be the best way to put the window into code? I'm in New York State. How can I find the codes for windows? Thank you.

Thanks ci_lantro. I appreciated the recommendation.

A cantilever requires a 2/3 to 1/3 ratio to start with. This needs much more than a bandaid of a Jack. It needs structural alterations that are going to require demolition and some rebuilding.

I didn’t mean to imply the window was out of code. What I meant was that the unsecured posts are dangerous and would not satisfy building code as permanent supports. They aren’t attached to anything at the bottom, I can’t tell if they are bolted properly at the top, the concrete base may not be big enough or deep enough to pass code, and one or both of the posts are out of plumb.

Thanks all for your input. Sounds that this will be more challenging that I expected.

Those aren't floor to ceiling windows, but that's off topic.

Using posts is a good solution, but they must be plumb and properly supported/secured top and bottom, which does not appear to be the case in the photos. That's a good project for a competent contractor.

Thanks all for the advice and recommendations.

was it put back to level when the jacks were installed?

To make sure, you are talking about the entire wall over the posts sagging, correct? So this wall is true between the posts? Or is it sagging between the posts also?. I think you may be overestimating how much it's actually sagging also. 2" to 3" is a lot and I would think if the outer wall were sagging that much you'd see things popping apart or separating from the house.