The derivative of intensity as a function of wavelength is dI/d(lambda). This can be approximated by Delta(I)/Delta(lambda).
If you have the raw data and can import it into a spreadsheet, you can can calculate the difference in intensity between one point and the next, then calculate the difference in wavelength between one point and the next. Divide the first by the second, and you have the derivative.
NOTE: Any noise in the original spectra will tend to get amplified if you take a derivative. You might want to smooth the data at the end of it looks too noisy.
Tim F