The stimulus questions are found in the "Federal Review" tab. Just click on that tab at the top of the page to go directly there. You can also type Stimulus in the search box and use the "Jump to..." link at the top of the search results.
Once there, you will be asked if you received any stimulus payments. If you answer "Yes", you will be asked to input the amounts. Your tax return will automatically adjust to reduce the Recovery Rebate Credit by whatever amounts you put in.
Here is a link with more information on the Recovery Rebate Credit.