Starting in the Layer Editor, clicking the newly added Update Layer button gives you the option of appending to the existing layer or replacing the existing layer's data. For example, if you decide to edit some features or delete some features, you can either bring back the original layer's data by replacing it, or you can merge the two layers, which would give you the combination of the original and whatever features you edited.

After clicking submit, you will be taken back to the layers tab in the admin panel, and you have to wait for the layer to finish processing in notifications in order to reopen it.