When you posted your question you indicated that you are using TurboTax Online. But you cannot use the What-If Worksheet in TurboTax Online, so I assume you are actually using the CD/Download TurboTax software.
At the top of the What-If Worksheet, above the four columns, on the right side there is a section that says "Clear amounts in column" followed by check boxes for columns 2, 3, and 4. Click the box for the column you want to clear. You cannot clear the current tax return in column 1.