I'd like to save my prepared return and revise after purchasing an IRA. How do I do that?

Everything in TurboTax online saves automatically. 

To enter your IRA contribution (from anywhere in TurboTax online):

1) Select Federal Taxes in the top menu

2) Select Deductions and Credits just below the top menu

3) Scroll down to Retirement and Investments and select Show More

4) Select Start next to Traditional and Roth IRA Contributions

Hope this helps.