After you complete the tax interview, you get a new text appearing on the same page that reads:
Do you need to make any changes to your tax information? Please contact support and tell us what kinds of changes you need to make and we can help you make updates.
That way you can update the interview details, or retake the whole tax interview.
Can I change the information after I send the form?
Yes, not only you can as mentioned above, but legally you have to update any information that changes over time.
if sometime in the future I want to add another person to the company(assume that I select solo in the form in first place) can I do that in my existing company I wrote in the form and in my current account?
If you’ve selected “Individual” in your tax interview, then I think you might need to change that into a company to add more people. I’m not too experienced with this, and it might be different in some countries, but I believe you do need a company to hire people.