I'm sorry for your loss.
To change your filing status:
Go to Personal Info in Turbo Tax.
Go down till you see Married Filing Jointly.
Click Edit to the right of the words, and then there will be another screen that says Let's confirm your marital status.
Check Widowed.
Answer What year did your spouse pass away?
Answer Yes to Do you want to file this return together with your spouse?
Click Continue.
You'll have to answer questions about your spouse on the next screen.
Click through to the beginning, and then continue your return.