I was referring to the top end of the sound spectrum, meaning the higher frequencies. You'll need to use an equalizer effect plugin.
The C* Eq - Ten Band Equalizer is a good place to start. Just put that in your effects chain along with two spectrum analyzers- one before the EQ and one after. Then, just play around with it for a while. The spectrum analyzers will give you a visual representation of how the EQ is changing the sound as you listen to it and help you figure out what you need to do. Generally I find it's good to try making the different frequencies about even with each other (which in LMMS usually means boosting the high ones), but it really depends on what you want to do/how you want things to sound.
The equalizers included with LMMS work, but they're a little unintuitive. If you feel comfortable installing VST plugins, there are plenty of free alternatives online. My personal favorite is Reaper's "reaeq" (which if you're using the latest stable release of LMMS should work just fine. Link if you're interested:
https://www.reaper.fm/reaplugs/ ).
Let me know if any of that made sense... I just realized I don't know what you'd consider your skill level...
