Assuming you are getting the same error I get opening Andy Pope's file (file extension not valid or similar), it is because, for some reason, whenever I download 2003 and earlier .xls files, the forum/my browser/Excel gremlins erroneously adds the .xlsx extension to the file, which does not match the file type. I find that I can open the file if I browse to the download folder, locate the file, rename file, and delete the .xlsx extension (you may need to change a windows explorer option to display extensions of known file types). Once the erroneous .xlsx extension is removed from the filename, then I can open the file in Excel just fine (in compatibility mode, naturally).

If you get to a point of opening Andy's file, you should see that he uses a fairly simple AND(this point>previous point,this point>next point) to test for the maxima (similar for the minima). The data he was working with appear to be essentially noise free, so this simple test seems to work fine. Your data seem a little bit noisier. I am not sure a simple test like this will work for your data. You can try it and see if you are satisfied with the results.

Any time I approach this kind of "signal analysis", I recall from calculus that maxima/minima occur where the slope of the function is 0, so the first thing I usually do is add a helper column to compute a slope for the data at each point. The details of this calculation will depend on how much, if any, smoothing I need to do on the data, or any other considerations around computing the slope. Once I have a column of slopes, then I will add additional helper columns to locate where the slope is 0 and changes sign and any other calculations I need. The final result is a lookup column that tells me where each of the maxima/minima are in the data and allows me to lookup or filter the data for those points.

If you are going to use this kind of signal processing approach, I would probably start with something like Andy Pope's solution. If I am satisfied with that simple test, then go from there. If I feel I need a better algorithm, refine the test as needed.

A possible different approach -- since you expect this to be damped oscillation (https://en.wikipedia.org/wiki/Damped_sine_wave ), I could see an approach that uses non-linear regression techniques (Solver) to "best fit" the parameters for the damped sine wave function. Your instructor may not expect you to know how to use this approach, or may prefer that you not use this approach. If you are allowed to, this would be a way of "fitting" both frequency/period and damped amplitude portions of the function simultaneously rather than finding them separately.