You'll need some form of audio editor (I recommend Audacity). Using this, open up your audio file, select all (ctrl+a), then open the Effect menu, and you'll see some bass/volume reducing effects: Amplify, Equalization, High Pass Filter,.
To reduce/change volume, select Amplify. In the pop-up window that appears enter the new value into the "Amplification (dB)" box or adjust the slider (lower to lower volume). If you find it will not let you go past a certain value, try unchecking "Allow Clipping". Clipping means that some sound information will be lost on the very loud parts, but sometimes you need to sacrifice this in order to make these loud areas softer.
To reduce bass quickly, you can select High Pass Filter from the Effect menu, then type in a cut off frequency (any frequency below this will be cut-out from the song). Usually "booming" bass frequencies are at or below 50Hz to 100Hz, so typing in a number in that range will work. Of course, since it just cuts those frequencies out all together, the end result might not be as nice sounding as the following method.
To reduce bass the effective way, select Equalization, then select the radio button that says "Graphic EQ". This will present you with a set of sliders that you may be familiar with from other equalizers. Here you can adjust the volume (in dB) of each frequency; a similar principle applies as above, lower the volume of the lower frequencies to reduce the bass sound, and if you want, you can also play around with the volume of the other higher frequencies as well. Generally, anything below 100Hz to 200Hz are the bass frequencies you might want to adjust, the others being mid-range and treble. Keep playing around with these volumes (especially in the 20Hz to 100Hz range) until you get results that you are satisfied with.
Other effects that perform similar functions to the above (and may be easier to use) are Normalize and Leveller. Explore these and the above and see which you prefer.
To apply the above effects to only certain sections of a file, select those sections and do the same as above.
I recommend starting off by finding those sections of the song that are too loud and adjusting volume/normalizing them (or the whole file). For "bassy" songs, I will alost use the GraphicEQ, starting at around 20Hz, slowly incrementing volume for each band towards 1000Hz. If its still really bad, and/or I get lazy, I'll just use low-pass cutoff around 100Hz or so (which usually kills the bass entirely..).
Here is a sample Graphic EQ used for the conversion of a bassy song:
Usually though, I find most songs can just be converted and played without editing, and only particularly bassy songs need special attention.
I recommend exploring all of Audacity's Effects; learning what each does and tweaking them to see what effect they have on your audio file. Once you find some combination that works, continue to use it (and feel free to give us a heads up here as well about what you discover!).
