Live concert recording fixing

I am completely new to audio so bear with me. I recorded a live set of some DJs using a H6 Field recorder. I was recording and I thought the levels weres spiking so I turned it down to level -18 db or in that are and then later in the set turned it back up to -3 in that area, but the first 5 or 10 mins of the recording are fine. My question is how do I correct the audio and the part where I turn down the audio, because in the waveform the audio fades down where I turned it down, so how do I get the audio back to a professional level?