I just filed my tax return and realized I made a mistake. When asked how much my first stimulus check was I entered $600 when actually it was $1200. I entered $600 for the second stimulus check which

If you filed, it is like dropping it into a mailbox - you cannot get it back, even if it's pending.

 

You'll have to wait until your return is either accepted or rejected when the IRS starts processing returns on February 12.

• If rejected, you can make any necessary changes regarding your stimulus payments.
• If accepted, you'll need to amend your return. 
• To avoid unnecessary delays, do not amend until your original return has been fully processed.  You will need to print and mail the amendment.  
• Allow 16 weeks for processing.

See this TurboTax FAQ for help with amending.

 

It is possible the IRS will process your return without rejecting it, in which case if you cash your stimulus check they will know and should automatically adjust your Rebate Recovery Credit, but it's impossible to know for sure at this point though.

Thank you very much for your help!