I think that it reads Ap. for April, and that the A in Ap. is very similar to the A in Aug on the line below.
I do not think that it reads Sep. because the S would look unlike any of the five distinctive examples of that letter from the same page.
I cannot be certain but I think the correction is written by the same hand using the same pen, and probably at or soon after the same time.
I think it would be useful to see if you can confirm that death date using one or more other sources like a death/funeral notice in a newspaper, if not a death certificate.