You have to unclamp the cable from the brake itself, remove the crimped piece at the end, and pull it back through the inline brake. Be careful not to let the end of the cable unravel so you can easily re-thread and re-use it, assuming it's still long enough after clipping the crimped end off.
Then, remove the inline brake lever from the handlebar (it has a clamp with an allen bolt). You'll be left with two cable housing segments where the inline brake was. Technically, you should replace those segments with a new housing of the proper length. I simply butted the two segments together (after re-threading the cable), taped firmly, and then rewrapped the bars. That portion of the cable (with the butted and taped segments) is under the handlebar tape and therefore totally secure and won't flop around or bend. I didn't have any issues with brake function. But if you have any doubt, just replace that part of the cable housing, it's cheap.
You then thread the cable back through the housings to the brake, re-clamp to the brake, put on another end crimp and adjust.