Yes, you can go back and change it.   The easiest thing to do is to just delete the entry you made with the totals, and then go through that section again.  Here's a shortcut to get back there, assuming you are entering from a 1099-B:

Log in, and at the top click on Search, then in the search line enter 1099-B without quotes.

Then choose "Jump to 1099-B".

It will likely show you a summary screen like the image below.

If you try to Edit it, it may just go through the Totals method again.

If it does, then instead of editing, click on the trashcan icon to completely delete it.   (see image)

Then go through that 1099-B interview again and choose to enter the separate sales.

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