Changing cells in previous rows based on new data

Kinda difficult to explain, but I was wondering if someone could help me out here.

I need either a floating toolbar/button (or frozen button at the top), that when clicked, will read the last row of data and compare it to previous data in the sheet. If certain cells in this last line of data match certain cells in previous rows, I want the contents of another cell in the previous row(s) changed.

For example with my attachment below, if I'm currently entering data on row 38, and I enter DEF in column E, 1987 in column F, and I am not putting this row "In Zone" (which would be a Y in column M. Column M will be blank on this row), I want to be able to hit a button that reads this bottom entry and compares it to the rest of the sheet (i.e, it would see that these two cells match row 27 and row 36). I then want the Y automatically removed from the column M from rows 27 and 36.

Note: I have the date running past line 38 intentionally. That column and other columns are irrelevant and shouldn't trigger the "search and destroy". I just need the very last row of column E and F that has content in it, to be looked at and compared to the rest of the sheet when the button is clicked.

I'm not even sure if this is possible or how I would even begin to get this right. Any help would be greatly appreciated.