Why? If you use the Online version you won't be able to efile it. What year? Don't you still have the program installed?
To amend a return online that you did somewhere else or in the Desktop program you would first have to fill out Turbo Tax like you are doing the original return and pay the fees for it. You have to recreate the actual wrong original return. Then when it matches each line that you really filed you have to tell Turbo Tax you will mail it so it thinks it was filed. Then you can amend it.
For the current year you can use the online version by first recreating the original return exactly as filed. Then pay the fees and say to mail it and print it out so it think you filed it. Then when it matches every line you can amend.