I have received a corrected 1099 composite from Schwab but I already imported the first one"

You can delete the wrong form and enter the corrected form. Please see the following instructions:

1. Login to your TurboTax Account 
2. Click “Take Me To My Return”
3. Click on the Search box and type “1099INT/1099DIV/1099B”.
4. Click on “Go to 1099INT/1099DIV/1099B”.
5. Click on “Delete/Trash icon” next to the 1099INT/1099DIV/1099B you wish to delete.
6. Click on the "Add" button to add a new form 1099.