My 2023 taxes are done so my details saved for 2024. I'm close to retirement & want to use TurboTax for what if calcs. How do I do that and not mess up my saved info?

[Opus 17]

Turbotax online isn't designed for that.  You would have to make additional online accounts with different user name and passwords, and enter your details all over again.  You can also try the TaxCaster app.  

TaxCaster tax calculator

To do what you want in Turbotax, you would have to buy the desktop version from a CD or download to install on your own Mac or PC.  Then download your 2023 data file, and make multiple copies (like having multiple copies of a word processor or spreadsheet document).  You can change the copies as long as you keep the unchanged original in a safe place.