Yes. TurboTax saves all your changes in real-time if you are using Turbo Online. In rare cases, you may lose your last entry or something you are currently working on if your computer suddenly freezes or you suffer a flash power outage. However, that would only happen in extremely unusual circumstances.
Because TurboTax Online updates in real-time, there is no undo. Once you make a change, the only way to undo it is to re-enter your old data.