First be sure to save your original return with another name so the amended return doesn't overwrite it and you can start over if it gets messed up. Go up to File-Save As and give it another name like add Original or FINAL to it.
If you don't see a button for Need to Amend a Filed Return? on your home screen then you can get to it here. Go to:
Personal (for H&B)
Other Tax Situations
last section, Other Tax Forms
Amend A Return - Click the Start or Update button
