In person, sources, a death certificate shows the birth date. The extracted birth date is incorrect. The edit feature does not provide the option to edit the birth date.
The index editing feature only applies to some fields of some indexes. We've been told that FS is working continuously on expanding the feature to more fields of more indexes, and we can see for example on the 1950 U.S. census that they're working on an "edit everything" version of the feature. Until one of those applies to the birthdate field of this index, you'll just have to console yourself that the index has already served its purpose, despite the error: you found the record.
For a relatively minor error like this, it's easy to just use the index entry as your source; you can make a note of the error in the notes/description field. If an index is completely botched and uncorrectable -- such as the ones in the Slovakia church books where the godparents got indexed as the parents -- then you can bypass the index and just attach the image directly as your source, using either the Source Box button or the image's URL.