Artificially improving the spectral power of a low frequency signal

First of all, I have to state upfront that I am not sure if this question is valid.

Essentially, my question pertains to the frequency of a time series with measurement data. I obtained the periodogram for the time series, where I have two peaks (see the left picture, around period = 12.5 and 25).

My question is: Would it be possible to 'artificially' increase the amplitude of the signal at the lower period (~ 12.5) such that the ratio between the amplitude of the larger signal to the small signal is decreased? My rough objective is shown on the picture on the right. What could I do to the raw measurement data to achieve this result?

I need to do this because I am trying to test a hypothesis on the measurement data.

Appreciate any advice. Thank you