If you're recording a live concert, you have to set your volume controls high enough to get well above the noise floor and low enough to avoid clipping. The first is easy, but no matter how hard you work to find the right level, you'll almost always end up with a few places in your audio file where clipping occurs. In particular, digital clipping is quite nasty. So you'll want to open up iZotope RX and select the Declipper module (it's the first module on the left). That opens the Declipper window. At the top you'll see a Presets box. If you click on the downward-pointing arrow on the right of this box, you can access several presets that come standard with iZotope RX. You might get lucky and find that one of these fixes your problem. You can try the presets in both Single-band and Multi-band modes. Most of the time the clipping you'll encounter is very brief, so Single-mode will likely work. A Preview button in the lower left of the Declipper module lets you hear the "fix" before you actually apply it. A help button (indicated by a question mark in the upper right) takes you to the appropriate window, which explains each function in the module. If you end up tweaking a preset, be sure to save it by clicking on the right-facing arrow on the right side of the Presets box. Choose "Add Preset" first, then if it's a variation of one of the onboard presets, continue by clicking on "Rename Preset" so you won't confuse the two.