I already sent in my taxes and forgot that I was receiving a K-1. Do I need to file an amended return?

[ee-ea]

Wait until you get you refund.

https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-federal-tax-return-curr...

[MinhT1]

If you have already filed your return, you'll need to wait for your tax return to either be rejected or accepted, if it is rejected you can add your Schedule K-1, correct any other errors on the return and resubmit it. 

If it is accepted, you will need to amend your return to include the Schedule K-1.

Amending won't delay your original refund. However, if you are expecting a refund, we recommend that you wait to amend until after the IRS has processed your original return and you have received your refund, feel free to cash or spend it while your amendment gets processed

Please see this TurboTax article for instructions on how to amend your tax return in TurboTax.

[nexchap]

To add to the previous answers, its possible that you're K-1 won't change your return.  You'll definitely need to enter it into TT so that its available for carryover to next year.  But the K-1 itself isn't sent in as part of your return so you only need to refile if it causes one of the submitted forms to change.